Difficult one.
I've considered moving on many times myself, and my thoughts on your choices are:
The MK6 is the perfected MK5 and a known entity, but not different enough to make the jump interesting, unless perhaps 4WD is a requirement. You'd have spent £15K, jump in it, and not a lot will be different.....compared to say jumping from a Golf to an M3.
MK7.5 is the perfected MK7, which has only just come out, so not financially viable yet unless leasing it. Early MK7s have the Gen 2 EA888 I believe, which has it's problems. Gen 3 EA888 is the one to have, but I'm not sure which model year it came out on. It's the one that'll map to 380ish if you stick the 7R's IS38 turbo onto it.
I've had 4WD Golfs and to me they feel too heavy to hustle around on back roads, plus you have the extra 4WD associated maintenance costs and weight penalty, smaller boot etc.
The only GTI that would make me ditch my ED30 is a MK7 Performance Pack.
So that would leave me in a position to keep my MK5
The ED30 is easily the best looking GTI since the MK2, it's easy and relatively cheap to fix and reliable if looked after. MK6s and MK7s are not different enough to justify the big cost jump imo, unless it's the 7PP with the Magnaride dampers and Haldex front diff.
That probably hasn't helped you in any way, but best bet is to test drive your shortlist and go with which ever you prefer