Something you might want to reconsider. When apps become more feature-filled and resource-heavy (and bloated) it's definitely nice to have a machine that's newer than six years old. Think about this: six years ago the top iMac was a 24" Core2Duo machine with a non-LED-backlit display, and the last OS it can now run is 10.6 Snow Leopard. That's not a pleasant way to run Adobe and CAD software (and the latest versions of those wouldn't run on that OS anyway).
I generally recommend keeping Macs with Applecare+ for the 3-year life of the Applecare coverage, then reselling the machine (which still will have reasonable resale value) and buying new again. Keep a machine for 6-7 years and you won't get much for it, you won't be able to run the latest most powerful apps and OS versions either. By selling when Applecare expires you will always have a Mac under Apple warranty, you will always have a recent-model that's cruft free and with the latest features/ports, and when including the resale value the cost for the running the sold machine usually ends up being between $1.00-$1.50/day.
Yep, everyones use-case is different.
Personally, the only thing I'd say is whichever model you have, upgrade the RAM to 16GB and try to avoid the fusion drives. They're still a marketing gimmick - when they come with a decent speed 256GB SSD they'll be worthwhile IMO, but the 128 is naturally limited.
$ for $, iMacs are not bad investments. With edu discount, I've just spent a total of £2600 on my new iMac - i7, 512GB SSD and 64GB of third party RAM (Apple's RAM is the worst investment you can make!).
I'm upgrading from a 2011. I'm parting it out, because it's been modified beyond believe so nobody would actually buy it as is (I know it's reliable, but you wouldn't buy one that's been modified, would you). So far, I'm at:
CPU: -100
Logic board: -120
PSU: -50
Stand: -50
GPU: -100
LCD: -150
Glass: -30
RAM: -150
Wifi card: -15
Misc cables and bits: -150
Rear housing: -100
Box (yes, someone bought the box): -30
Headphones that came free (edu discount): -250
That's £1295 back in various bits and pieces, for a few hours ebaying, and I've still probably got a couple of hundred worth of bits left. That makes the net cost of upgrading 1305. Over 5 years (what I tend to keep them), that works out at 71p per day. It probably costs more in electricity to run it than that.
Over 5 years, I only need to do 3-4 days of consultancy using it over the next 5 years (VMware consultancy pays well, and my main reason for upgrading was so I could have 64GB RAM which will come in useful for testing vsan environments), and it's basically paid for itself.
Could I justify that £2600 if I was just using Facebook - no way. But for me, it's an absolute no-brainer.
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Yep...your 2012 can work for a while. The longer you wait, the better you'll do for the money spent in most cases. Just try not to get caught up in the nonsense. I have a 4K sitting next to a 5K and I can see a small difference in a few sources, but it's really not significant. I also have SSD, fusion and standard drives and I'm a professional photographer working with monster-sized RAW files in batches. Everything opens fast and it all works great on these machines.
Over the last few years we've hit a technological plateau where things got very fast, but the advances have become less and less meaningful beyond those editing high res footage or doing a lot of gaming. My 2011 i7 iMac could do just about anything I threw at it and my clients were always happy. Do my machines work faster in a way that improves workflow? Hmmmmm. To be honest, no. And my friends here in LA who edit feature films? Nope. They don't even work off internal drives and when they do, even a spinning drive is still fine.
So don't get caught up with folks who seem to define themselves by telling people what they need. I just saw a post where a guy said his SSD opens his RAW camera files instantly. Good grief. My 2011 machine did that. Maybe he means his SSD will open a batch a half second quicker. What does that mean in real world workflows....ZERO. There are some savvy people here, but the majority are not using their machines for task intensive applications. Frankly, the i7 5K 27" with 580 card and 2TB is overkill for me and I'm shooting with the best DSLRs on the planet. I plan to upgrade it down the road with a much bigger internal SSD...but I probably won't really need to! But people love to think they're buying the "best" even if they really don't need it and will never use it. Most folks who buy a fast car don't drive it fast or know how.
R.
Again, it all comes down to use-cases.
For me, the speed of the new SSDs is a huge benefit. I spend half my life provisioning/installing vms. When I went from HDD to SSD back in 2011, it was a massive benefit. Now I'm going from 2 SATA SSDs in raid to the new PCI-e SSDs. That could easily halve the amount of time I spend provisioning, which over the course of a day, could be a good half an hour saved.
Doesn't sound like much, but over a year, those half hours really add up.