I did prefer GP5, but now that I have used GP6 for a couple of years I have got used to it. Here's my view:
Things I prefer on 5:
Drum tabbing is MUCH faster. I honestly don't know why they changed it.
It sounds better unless you spend time tinkering. I don't like how GP6 has a bunch of DAW-esque features that require you to adjust levels and build tones to get a passable sound. The MIDI sounds of GP5 was much clearer and faster, and meant you spent more time writing than getting it to sound passable.
More stable, GP6 seems to crash at least once a day.
Things I prefer on 6:
Has the potential to sound a lot better than GP5, although I don't really care about this as I don't need it to sound good, I need it to write ideas.
Global pitch shifting. Sounds small, but I have found this quite useful.
Transposing, I have found this very useful for changing things across multiple tunings.
Final comment, if GP6 had GP5's drum tabbing and didn't try to be a DAW then hands down it would be the better program.