Well, first question is, why do you need to upgrade? 2014 i7 iMacs were more than adequate for line art and 2D graphics, they're still supported by the latest OS, so unless they're faulty or you have some business reason (warranty/leasing/tax) for upgrading now - or are just craving that 'new computer' smell - it wouldn't be a high priority.
Yes, the newer machines are somewhat faster, but its not going to be a game-changing improvement.
Apart from that: x2 on what @
Fishrrman said about RAM and SSD.
The 3.0/4.1GHz CPU should be fine for you - I doubt that you'd notice the extra 0.1GHz (or the slightly better GPU) on 2D work - put the money towards getting a 256 or 512GB SSD - the machine will start up faster, applications will load faster and if you
do run short of RAM the SSD will soften the impact.
If 256 or 512GB sounds a bit small, use external storage to extend it - maybe think about sharing a big "Network Attached Storage" drive between the two computers for archives/libraries etc. (or, maybe, cloud storage, but I'm old-fashioned) You'll need some sort of external storage for backup purposes anyway. It's really the system and applications that benefit most from the internal SSD.
Friends don't let friends buy the 1TB Fusion drive (which comes with a pathetically small SSD cache). If you really must have 1TB+ built-in storage, either grit your teeth and fork out for the 1TB SSD or at least go for the 2 or 3TB Fusion drive (which have bigger SSD portions).
The 5k iMac has
very easy-to-upgrade RAM (no screwdriver needed) so there's no need to 'future proof' by buying more than you need. Run 'Activity monitor', ignore "memory used" (MacOS uses all the memory it can to cache things) and look at "Memory Pressure" and "Swap used" - if the former is in the green and the latter is 0 you're OK for memory. If not, buy a 16GB upgrade kit from someone like Crucial and you'll have 24GB RAM for less than Apple want for an upgrade to 16GB.
