In my experience P7 is very stable. It renders faster and has new figures that look very nice IMO. It has multible undo's and that can lag the program. However you can set the maximum undo's to as low as 1 or 2, and there will be no more lag.
I would recommend P7 if you have a decent system and enough memory (like atleast 1 gigs or more).

-Morbo will now introduce the candidates - Puny Human Number One, Puny Human Number Two, and Morbo's good friend Richard Nixon.
-Life can be hilariously cruel