I think fed in 17 was better - but not by a big margin (better groundstrokes, didn't have problems with keeping up level when matches went for more than 2.5 hrs or so vs better movement in 15, better net game and a very marginally better serve).
And importantly, he played more freely in 17 -- including vs Nadal, was better mentally. don't see fed 15 being able to do that vs nadal 17.
Also going by surface speeds :
fed did well on grass in 15. same in 17 (Halle and Wimbledon)
did well on medium-fast to fast HCs (cincy, USO, brisbane) in 15, same in 17 (AO, shanghai, basel)
but he did better on slow HC in 17 by a big margin (IW 17, Miami 17 compared to IW 15, AO 15)
he did play clearly better at the YEC in 15 compared to 17 though.
AO 15 djoko vs AO 17 fed would be close on the 17 court. I do think djoko would probably outlast him on the 15 court though
Wim 15 djoko vs wim 17 fed would be close as well. overall I'd favour wim 17 fed by a bit, but final form djokovic would be favored by a bit.