Reddit mentions of OWC 64GB (2 x 32GB) 2666MHz DDR4 PC4-21300 SO-DIMM 260 Pin Memory Upgrade, (OWC2666DDR4S64P), for 2019-2020 27 inch iMac (iMac19,1 iMac20,1 iMac20,2) and PC laptops

Hey there,

hope I can help you a bit in the following. The selection of devices you have in mind is already very good, I'd say, all those machines are great in their own right. So I imagine that it is hard to choose between them... Since you are a developer, and since you didn't say anything about GPU-heavy tasks like gaming, video editing etc. being a thing, I think we can do without a comparison of GPU power here, but I can still do that later on in case you want me to, for video editing, gaming etc. purposes.

I think as a very first step, it makes sense to take a look at the Geekbench scores of the respective devices, so that you can get an idea about the performance difference between them. I'll also include the current MacBook Pros 15" 2019 in the comparison. Here we go (higher is better):

MacBook Pro 13" 2018, 2.7 GHz Intel Core i7-8559U Quad Core:

5141 Points (Single-Core Score)

17783 Points (Multi-Core Score)

source: https://browser.geekbench.com/macs/433

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MacBook Pro 15" 2019, 2.6 GHz Intel Core i7-9750H Hexa Core:

5260 Points (Single-Core Score)

23068 Points (Multi-Core Score)

source: https://browser.geekbench.com/macs/445

MacBook Pro 15" 2019, 2.3 GHz Intel Core i9-9880H Octa Core:

5435 Points (Single-Core Score)

28396 Points (Multi-Core Score)

source: https://browser.geekbench.com/macs/446

MacBook Pro 15" 2019, 2.4 GHz Intel Core i9-9980HK Octa Core:

5589 Points (Single-Core Score)

29854 Points (Multi-Core Score)

source: https://browser.geekbench.com/macs/453

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iMac 27" 2019, 3.7 GHz Intel Core i5-9600K Hexa Core:

5796 Points (Single-Core Score)

22990 Points (Multi-Core Score)

source: https://browser.geekbench.com/macs/439

iMac 27" 2019, 3.6 GHz Intel Core i9-9900K Octa Core:

6293 Points (Single-Core Score)

33961 Points (Multi-Core Score)

source: https://browser.geekbench.com/macs/438

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iMac Pro 27" 2017, 3.2 GHz Intel Xeon W-2140B Octa Core:

5072 Points (Single-Core Score)

31359 Points (Multi-Core Score)

source: https://browser.geekbench.com/macs/426

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As you can infer from the scores above, there are significant performance differences between the individual devices. The MBPs 15" have more physical cores than the 13" models - 6-Core i7 or 8-Core i9 in the 15" models vs. 4-Core i7 in the 13" model - and thus have a much higher Multi-Core speed. The Multi-Core Score is especially relevant, as most modern applications are designed to make use of multiple cores all at once. Thus, a higher Multi-Core Score directly translates into better general performance for the most part. The Hexa-Core i7 in the MBP 15" 2019 base model is a clear step up from the Quad Core i7 of the 13" model. The i9 Octa Core chips in turn are both a step up from Hexa Core i7 even, though between the two i9 chips of the MBP 15" 2019 there isn't much difference, so if you can get a model with 2.3 GHz i9 cheaper, I would rather get that.

The iMac 27" 2019 with i9 Octa Core chip is a step up from the i5 Hexa Core for the same reason, 8-Core i9 vs. 6-Core i5, thus far superior Multi-Core performance. The iMac 27" 2019 with i9 chip even outpaces the iMac Pro 27" 2017 with Octa Core by a slight margin, this is important to remember due to the fact that the regular iMac 27" 2019 with i9 chip can be bought for less money than the base model iMac Pro.

All in all, I think it's clear that the i9 chips are the top notch performers by far, no other CPU in the lineup can touch the performance of the i9 chips even remotely. The i9 in the iMac 27" 2019 being the best performer of them all.

Now, the matter of RAM: If you opt for a MacBook Pro, I would opt for 32 GB RAM (15" models) or 16 GB RAM (13" models) if you want to future-proof them, maxing out the RAM absolutely doesn't hurt in any shape or form, it enhances the longevity of the devices in question, and ensures that you will not experience slowdowns related to a lack of RAM. The RAM in the current MBPs 13" and 15" is soldered in, sadly, so you can't replace or extend it later on. Your decision at the time of purchase is the final one for the lifetime of the device, so I'd say better safe than sorry in this case, as there is no way to fix a lack of RAM later on with these machines.

A definite advantage of the iMac 27" 2019 is that its RAM is user-serviceable and that you can therefore easily upgrade it yourself. The iMac 27" 2019 supports up to 128 GB RAM (4 x 32 GB RAM modules). It has 4 RAM slots, meaning 4 RAM modules in total will fit in. A higher amount than 128 GB RAM won't work due to a limitation in the logic board.

The iMac 27" 2019 needs RAM that matches the following specification:

PC4-21300 DDR4 2666 MHz, 260-pin

Here are some examples of RAM modules that work flawlessly in the iMac 27" 2019:

• https://www.amazon.com/Timetec-Compatible-27-inch-Display-PC4-21300/dp/B07MH3KHLY/
  
• https://www.amazon.com/CORSAIR-Vengeance-Performance-260-Pin-CMSX32GX4M2A2666C18/dp/B01BGZEVHU/
  
• https://www.amazon.com/Crucial-Single-PC4-21300-SODIMM-260-Pin/dp/B071KP8CGJ/
  
• https://www.amazon.com/OWC-2666MHz-PC4-21300-OWC2666DDR4S64P-macmini18/dp/B07PTRZSHB/
  
  Take the Crucial RAM sticks I linked to, for example: 16 GB RAM = $64... 4 x $64 = $256. Same RAM upgrade from Apple: $1000. And yes, as long as you remain within the specification above, you get the exact same RAM technically that you would also get from Apple, no technical difference whatsoever. You'll get around $750(!!!) back.
  
  The RAM upgrade is extremely easy to do, take a look:
  
  
• https://www.youtube.com/watch?v=PuXKiFyzRzs
  
  The ability to upgrade the RAM yourself in the iMac 27" 2019 is a huge plus, as you've just seen. The iMac Pro 27" 2017 also has user-serviceable RAM. Sounds great? Well, you were rejoicing too early... The RAM in the iMac Pro isn't soldered in, so the theoretical option to extend it yourself exists. But it's mostly theoretical, as you have no easy way to physically access the RAM in the iMac Pro. Extending the RAM yourself in an iMac Pro requires a partial disassembly of the device, including unglueing the screen from the device body and taking the logic board out. The procedure is a mess, take a look:
  
  
• https://www.youtube.com/watch?v=_fBVy26FNbE
  
  Furthermore, the ECC-RAM that the iMac Pro needs is fairly expensive compared to the RAM the iMac 27" 2019 needs, even if you buy third party RAM.
  
  As for ports: The iMacs come with more classical ports, among them USB-A, SD-Card reader, and Ethernet. They have a clear advantage here if you need any of the aforementioned ports regularly, because if you have equipment that uses those ports and own a MacBook Pro which only has USB-C, you won't get around using a dongle regularly. The iMacs have a better mixture of legacy ports and future-proof USB-C ports, while the MacBook Pros went all in with USB-C.
  
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  Which machine would I buy if I were in your shoes? I think I'd buy the iMac 27" 2019 with i9 chip, it's the most powerful machine out of the bunch in terms of processing power, it has the same screen as the iMac Pro, the same ports as the iMac Pro (well, it has two USB-C ports less, but still...), and most importantly, you can upgrade the RAM easily in this machine and save tons of money that way. Make sure you configure in an SSD at the time of purchase, though, a Fusion Drive just doesn't do this machine justice. Add the 512 GB SSD to it at the bare minimum. The iMac Pro is IMHO not worth it compared to the iMac 27" 2019 with i9 chip, as it's a bit slower, has far less accessible (and more expensive) RAM, and is on top of that more expensive by itself already.
  
  In case you opt for a MacBook Pro, I'd opt for an i9 model here as well if I were in your shoes, as the i9 chips are much more powerful than the others. Less upgradable machines and worse port selection compared to the iMacs (be prepared for dongle life...), you also lose the gorgeous 27" 5K display... You gain mobile, though. If the "mobile" factor isn't of extremely high importance to you, I'd head straight for an iMac, you get far more bang for your buck in this case.

Yup, upgraded both memory stick and SSD. I think there is a backlag when ordering X1 Extreme Gen 2 since this laptop got so popular now that Lenovo is having difficult time keep up with the demand, but definitely recommend get lowest spec you can for SSD and RAM and you can upgrade them yourself. Just makes sure you get the right screws.

In case if anyone is interested, below are the SSD and RAM I got and so far my laptop has running amazingly well and memory utilization rarely exceeds 60%.

For SSD, I use Samsung 970 EVO Plus 1TB

For ram, I use OWC 64GB (2 x 32GB) 2666MHz DDR4 PC4-21300