Which 2020 27-inch iMac model is best for longevity?

No matter which variant of the iMac (Retina 5K, 27-inch, 2020) you are talking about, it will

• Never receive Apple Intelligence
• Never be able to update past macOS 26.* (Tahoe)

And that's the only 27" iMac that can update as far as Tahoe.

Apple has completely transitioned their product line from Intel-based Macs to Apple-Silicon-based Macs. The current version of macOS, macOS 27 (Golden Gate), will not run on any Intel-based Mac.

If you are looking to replace a 21.5" iMac with a Mac that has a 27" screen, your best bet might be

• A M6 Mac mini or M5 Pro Mac mini paired with a monitor, keyboard, and mouse. There are many 27" displays which you can choose from, including some third-party ones that have the same 5120x2880 resolution as the Apple 5K Retina iMacs and Studio Displays, but that are much less expensive than the Studio Display.

There is a 24" M4 iMac. It has a nice screen, but it sounds like that screen is smaller than you like. Also, that iMac is based around an older-generation Apple Silicon chip whose graphics are not nearly as fast as those of the M6 chip in current Mac minis.

Note that RAM is not upgradable on any Apple Silicon Mac. If you want your Mac to last a long time, I would not recommend getting any Mac with less than 16 GB of RAM. If you plan on running heavy-duty photo processing software, or doing extensive video editing, you should get more.

On a desktop Mac, you can probably get away with 512 GB of internal storage, if you are willing to add external drives as needed, but life might be easier with at least 1 TB of internal storage.

My organization has a small number of the later Intel iMacs and I do not recommend them since several of ours had bad Logic Boards. Except for the old white 2006 iMacs, I've never had to replace the Logic Board of any 2007-2015 iMacs until we started using the 2017, 2019, 2020 models. Keep in mind the 2020 model is a T2 iMac.

In my opinion the better option would be to get an Apple Silicon Mac Mini and an external display. The initial cost will be less than a comparable iMac plus when the computer needs replaced, the replacement cost will be much less since you can re-use your external external display. Plus we've seen people report display related issues with some of the Apple Silicon iMacs which another reason to avoid them. The Displays on iMacs have been hit or miss over the years and it seems this trend still continues.

No Intel Mac is going to have much of any future at this point. You should only be looking at Apple Silicon Macs if you are interested/concerned about longevity.

That 2020 27" iMac is ...

• Intel. Sequoia & Tahoe are the only current versions of macOS that will run on an Intel Mac. Both will be "off" support within 2 years. Golden Gate and future versions on macOS do not run run on Intel Macs.
• Already 6 years old.
• Undoubtedly used ... and you run risks purchasing a used Mac from anyone other than certain known good vendors Apple Refurbished or OWC. Purchasing from someone on eBay or even Amazon is high risk for being improperly prepared for resale, resulting in *your* loss.

non of them really

apple never made an imac with their own arm based cpu series, and they no longer support intel cpus with their latest macOS version so at best intel based macs will still receive security updates for a while, but likely fewer and fewer apps will support the older cpu models

I doubt the OP is going to return, but I'll add my 2 cents anyway.

None. Intel based Macs are done for. Don't waste your money.

To add on to Servant of Cats response consider the following:  a 12‑core CPU, 12‑core GPU Dual 16‑core Neural Engine Mac Mini M6 with 16 GB RAM and 1 TB SSD ($1399 US) with a 32" LG 4k monitor ($331 US) from Amazon.com)  is $1716 which is $480 less than a similarly configured new 24" M4 iMac ($2199 US). Keep your keyboard and mouse from your current Mac for use with the Mini. If you recycle don't include the mouse an keyboard.

I got the previous Mini M4 with a 32" LG 4K monitor w/speakers for the same price as a 27" monitor (didn't know the Mini had a speaker).  There are many monitors available in 27" size for about $100 US to $500 US depending on what features you want.  I love the extra screen real estate of the 32" monitor.

I had an 2017 i7 iMac with an SSD boot drive and a new M4 Mini runs circles around it.  The new M6 will be even faster.   

You can use the keyboard and mouse from your current iMac. Don't include them with the iMac if you use Apple Trade In or recycle with the  iMac.

Just some food for thought.

: the only way to get an all Apple 27" Mac is to get the Appe Studion 27" monitor which starts at $1599. Granted it is a 4K monitor but you can get 5K third party monitors for about ½ that.

OldToad wrote:

I got the previous Mini M4 with a 32" LG 4K monitor w/speakers for the same price as a 27" monitor (didn't know the Mini had a speaker).

The Mac mini and Mac Studio have speakers – but as far as I know, only monophonic ones. If one listens to music or watch TV shows or movies, some sort of stereo speaker arrangement would be better.

It doesn't have to be one built into a monitor, since USB "sound cards" and powered stereo computer speakers are available.

Servant of Cats wrote:

OldToad wrote:

I got the previous Mini M4 with a 32" LG 4K monitor w/speakers for the same price as a 27" monitor (didn't know the Mini had a speaker).

The Mac mini and Mac Studio have speakers – but as far as I know, only monophonic ones. If one listens to music or watch TV shows or movies, some sort of stereo speaker arrangement would be better.

It doesn't have to be one built into a monitor, since USB "sound cards" and powered stereo computer speakers are available.

You're right. The monitor speakers don't support surround sound nor does the Mini. I use a pair of AirPods Pro 1 when ever I want to listen to music or watch a movie or clip. My AirPods Pro 2 are used with my iPhone for listening to audio books.

I had a very cheap pair of bluetooth speakers that sounded better than my monitor's speakers. And they supported surround sound. After 3 moves they got lost.

CORRECTION:

1 - "Granted it is a 4K monitor" should be "Granted sit is a 5K monitor"

2 - ": the only way" should be "NOTE: the only way"