no comparison between the two. imo, if you aren't going to spend the money for the f8500, you might as well go with the pannies(st60 maybe?)
input lag isn't that bad when set to PC mode. it's on par with most other high end displays(the high end stuff never seems to have less than about 50ms... too much processing I guess?) but is quite bad in the normal mode. in either event, it's only relevant for input sensitive activities(gaming, using as a computer monitor...) and has no effect at all on normal video watching(tv, blurays, etc)
the top 4 for picture quality this year are the f8500, st60, vt60, zt60. it really pays to get a lot of hands on experience with all 4 before you pick your personal winner though. I didn't personally see a huge difference between the 3 panny's, all very good, very black, very safe bets. the Samsung stood out for sure, more LED looking than any other plasma I've seen. it's up to you to decide if that's for the better or not though.
Displays: Samsung PN64F8500/JVC X35
AVR: Pioneer VSX-1018AH, 5.1 audio
Sources: HTPC(Mediabrowser), PS3, XBOX360, Wii, Sony DVP-CX995V
Control: Harmony One