Barca advantages:
More individual skill AND team creativity - Xavi, Iniesta, Messi, Pedro combo is special...chemistry like I've never seen before...and there's nothing to stop Messi on his day.
More imaginative team play overall
High press defense - If they get that right on the day, forget it - Man U doh have the ball handling genius to deal with it.
Barca weaknesses:
Lack a bit of killer instinct - let teams off the hook, take their foot off the gas when the game seems comfortable, and sometimes a bit too laid back in how they take risks with the ball in dangerous parts of the field (particularly in the middle and at the back)...
Tactical naivete? - Not sure about this but get the impression that Barca expects to outplay the opposition, and not necessarily try to out-coach...so in a tough spot, I dunno how tactically flexible they are to switch up their approach...but then again there's always Messi.
Man U advantages:
Crowd support - It will be closer to a home game for them as they will have most of the neutrals and probably a bigger traveling fan base
Discipline & Tactics - They play a less risky and more disciplined brand... lower chance of mistakes. Fergie will also put alot into the tactical battle and make it difficult. I give him the nod over Pep if it comes down to tactics.
Man U major weakness:
Overall lack of creativity and individual brilliance to lift them if they're having an off day... This is the most basic, fundamental playing Man U team I've ever seen...a blessing and a curse depending on the scenario.