Way back when I started this thread to help me decide which one to buy. After a lot of hemming and hawing I picked the 3.5i5/575/1TBSSD/8GB. It finally came Wednesday. I added 32GB of RAM, fired it up, and have been using it happily since. Here are my observations for what it's worth.
Exactly what I have.
I have the nagging thought that since I can hear it all the time anyway, maybe I should have just gotten the i7 anyway. If the thing is going to have that whir sound at idle anyway, you might as well have the horsepower when you need it, right? Regardless, I got by on half the speed for the last 5+ years, so this is all the CPU I really need.
No. The ramp up of the fan means much more noise, and it can happen not just with hardcore video encoding and such, but occasionally some more moderate usage as well. Infrequently, yes, but if you're sensitive to noise it would be an issue for you.
I tried to convince myself of that, but in the end I'm happier with the "slower" machine. YMMV though.
Yes, I can get up and let the machine render while I go get a coffee, but sometimes I don't want to do that. With your iMac, you can continue to use your iMac with the same low fan, but with the 7700K, it will be loud.
The screen is a bit high for me. I'm only 5' 6" and when seated comfortably at my desk, the iMac is a bit too tall and neck strain is the result. I really wish you could move it down a few inches. In fact, I wish it enough, that I will probably either replace my desk or modify my desk to lower the stand base 2-3 inches.
I'm 5'7". I ended up buying a VESA mount for my 2010 iMac and lowered it by about 2.5 inches. However, it was a pain because it was not as 100% stable as the iMac stand, and having the iMac so low meant anything on the table got in the way. The main problem is the stupid chin there. Plus I wonder what it would do to fan noise and fan effectiveness in the current models, since the fan vent is at the bottom.
Plus, as you can see, it wasn't very pretty.

I just wish they could somehow come up with a way to eliminate that chin, even if it meant making the iMac thicker.
I had contemplated just getting the smaller iMac, but the value just isn't there if you want to get lots of RAM and a big SSD. It's unfortunate though, since its height is perfect for us short people. Plus, the 27" is a match for my old one, for a dual iMac configuration.