We did Epcot last NYE. I believe Illuminations had a few extra twists to it. It was a specatcular show. We watched from near the Rose & Crown restaurant (England), but there are many good spots. People were though "camping out" for hours before the show along the lake, reserving their spots for the show. We chose Epcot since it's so big and we wanted to be able to "move around" a bit (MK gets ridiculously crowded). Even though Epcot is big, 1/2 hour before the show it was standing room only around the lake, and you basically couldn't move anywhere until after the show.
Can't comment on the PI fireworks