What to choose imac 2019 or 2021

Either will last 3+ years. Even if Apple doesn't offer the newest OS for your machine next year all the software you list will run on any of the four newest versions of MacOS.

The 2021 M1 has better performance for CPU based image processing and being newer is more likely to last the full 5 years.

A 2019 (27" 5K) is the only option with user upgradable RAM. It also has better performance for GPU based image processing (Topaz?).

If your budget was for a 2019 21.5" 4K then 100% skip that and get the 2021 M1.

One significant difference between the Intel iMac and the newer Apple Silicon M processor iMac is the older Intel iMac has a dedicated video card processor with its own dedicated memory, the Apple Silicon iMac has separate CPU and GPU processors that share the same unified memory. So, RAM comparisons between the two are not quite what you expect. Think of having a little more RAM than you are accustomed to to account for having enough to share with the graphics processor on the newer Silicon Mac. Maybe 1-2GB or so? Multitask everything you are doing now on your iMac and see what your memory usage is with /Applications/Utilities/Activity Monitor.app, then add some extra for graphics to determine how much unified memory you might be using on a Silicon Mac. Just a starting point. I would check that RAM use as a guideline, then double it, to allow for future-proofing, as much as you can with a product that has imminent obsolescence as a part of its inherent design and marketing. :-)

Also, how much free space do you have on your current laptop, it may be freezing due to lack of free space on your hard drive.

https://www.apple.com/uk/shop/product/FGPK3B/A/Refurbished-24-inch-iMac-Apple-M1-Chip-with-8%E2%80%91Core-CPU-and-8%E2%80%91Core-GPU-Gigabit-Ethernet-Blue

You'll compromise slightly on RAM and SSD (8/256) but both IMO are fine with responsible use (defined as not subjecting your computer to too many accounts, open browser tabs, open apps, multiple displays etc.).

FWIW, I have never bought more than the bare minimum including CPU and have never regretted it.

If you need more SSD, you can get 512 for about 140 quid more. Have a nice day!

8GB is the bare minimum for image processing. Even with the ability to perform memory swap at SSD speeds (which your MBA also does) I would prioritize at least 16GB.

As a third option you might consider getting a Mac Mini and bringing your own mouse/keyboard/4K monitor. Even new M2 Minis with 16GB RAM should be in budget, and while internal storage is preferred you can use cheaper external USB-C SSDs if/when needed.

Just to add, 16GBs of RAM on an Intel Mac and an Apple silicon Mac are not the same. Silicon Macs can use part of the SSD as additional memory. I would stick with a 512GB SSD if possible. The 256GB SSD is not just smaller but slightly slower.

Sorry. My fault. I missed the "2021". I would still opt for the newer M1 iMac.

I would go for the latest, the M3. I would never go for an Intel Mac. The difference is so big with apple silicon and the Mac is performing so smoothly. An Intel Mac is a bad deal now that apple silicon is avaliable in my opinion

I own a 2019 27" iMac (I9 processor) with 1TB drive and have added so my Ram is 24GB. I also have an M1 Mac studio with 32GB RAM. Both function about the same. I see little speed change between the two when I render videos. My Mac Studio boots faster but is also more finicky when using external drives. Both run Sonoma 14.1.2.

Just to add, I wouldn't buy either from a private party. Only a refurb from Apple or a reputable source.

The two aren't comparable. Specifically, laptops will throttle the CPU more often because they get hotter faster. And because laptops are cramped for space, heat ventilation is physically limited by design, so throttling is more frequent.

Desktops don't have the same issue. Furthermore, RAM is more of an issue with older-generation hard disks where swapping slows the computer down markedly. OTOH, the Mac SSD hard drive there is the fastest there is so swapping won't get in the way.

You also have the option of trying it out within the return window and also buying Apple Care+ post some point after the return window to future-proof your purchase. I'd go for it.

I would opt for the M3 iMac with 16GBs and a 512GB SSD.