I've played PES2008 at GC, both PS3 and Xbox360 versions. Visually they look the same, with no slowdowns at all (finally!!). And when you see gameplay of both new FIFA and PES, you won't notice huge difference except of better presentation of FIFA (better menus, replays, and so on). BTW funny to see FIFA becoming more and more PES-alike

Even the controls of new FIFA are nearly identical to PES's.
But since I don't play FIFA at all, I've spent my time playing new PES. What I noticed immediately:
1. New animations for player movements/dribbling/shooting. First impressions - they are much smoother than at PES6. But a bit slower (which isn't too shabby at all I must say - I wasn't huge fan of animations in PES6).
2. AI-controlled players defend MUCH better than in PES6 (I haven't played vs AI team though). Personally I use "doggy-style" defense in PES6 - holding "shoot/attack" button and trying to predict with my player where the ball will be next time, while AI-controlled player from my team is attacking the guy with the ball. It's not necessary now - AI-controlled players move a lot better without a ball, I even had a few situations where they performed double attack themselves!!! and took posession of the ball while I was thinking what to do and where to run. That's really nice - because you have more time to plan counter-attack now

3. Same goes to attacks - players move a lot better without a ball, for example: I hate situations in PES6 when I take the ball in middle, press LB+X then dashing onto the goal and expecting previous player to run in parallel. He runs for a moment, then stops. I assume he was expecting return pass but I wanted to make the return pass much later, when I'd attract keeper and defender. This really pisses me off!! But in PES2008 it's fixed - now when you press LB+X previous player will run until he's either in deep offside or receives the ball back. I scored couple of times with this tactics - works really well!!
4. Main menu and in-game pause menu are new and pretty damn good, while formation settings menu is same as in PES6. I'm really tired of it, don't know about you guys, but the fact is that formation settings menu is the SAME.
5. Overall graphics are a little bit better than PES6 but still it's not a huge step forward. Players models are new but not revolutionary better, pitch textures are also just a little bit better, crowd is pretty much the same.
6. Back to gameplay - it's now much harder to dash through the flank since AI makes reaaallly nice "shifting move" - when you run on flank and opposing side defender will attack you, central defenders will automatically shift to the flank, backing up side defender in case you'll go past him. Looks fantastic - just like in real football!!
7. There are a lot of new dribbling moves - but I can't tell you much about them cos I couldn't make them

Just saw guys from Konami made some sweet dribbling moves when playing in front of TV camera. I assume dribbling controls are all new.
Overall, I was rather pleased with what I saw, because it's finally new PES - not like PES6 comparing to PES5

Still I have a lot of questions about AI - will it really be adaptive and self-learning or it's just another marketing bulls**t. Not to mention ML - guys from Konami told me that it's "still in development and we can't really talk about that", which shows IMO that it will be the same as in PES6. Otherwise Konami would show it somehow, maybe even on videos!!
Ok, hope it wasn't a tough read for you guys since English isn't my native language