Buying tech is NOT an investment. Unfortunately politicians, in a move to avoid using "spending" or "taxes" have dumbed up the word to make spending on odd items more palatable to voters. Wasteful spending is wasteful spending though no matter the slick lexicon used to describe it. What is wasteful spending? Buying something you don't need.
Mac's hold their value very nicely. I have a 2012 entry level 21.5" iMac that I paid $1K for new. I can get about $500 for it now -- 5 years later. That's better resale recovery % than a car. So my advice buy what you need for today. When/if your needs change then re-access and buy what you need then. You'll be able to get a good chunk of $ out of whatever you buy now.
Case in point, my 21.5" iMac. I bought it to tide me over back in 2012 as I was waiting for a new MP (which, of course, was long to come and when it did was very different than what MPs were in the past). It has a lousy 8GB RAM and an integrated GPU. But in 5 years, after a lot of video editing (mostly 2D) and picture editing with Lightroom, along with occasional Photoshopping (CS5,6) it really wasn't that bad of an experience -- to the point I never stopped one day and said "I need a new computer".
I'm ready now and will be getting an entry level 27" and popping in 16GB of RAM myself. I'll sell the 21.5" to defray the cash outlay. Total expenditure should be around $1400 when done. Not too painful at all.
So if you are trying to be fiscally responsible buy the machine you need for today and worry about tomorrow when tomorrow comes. Save the money you would have spent buying the faster machine for use toward you next machine. Odds are that will be 1, 2 or more years away. If you bought more than you needed now that will have been wasted and not all that fast compared to the current models by then.