I prefer the 3-4-3 (for 95% of our games), because it allows us to give all our midfield-stars enough playing-time.
Those first 30 minutes yesterday were pure sex, awesome to watch. Against Villarreal our 3-4-3 looked still quite rusty at times, but yesterday in the opening half hour the passing preciseness was so good and we found so many openings, great to watch.
It seems logical to employ a 4-3-3 against the top-sides with great wingers (like ManUtd and RM), but even in those games, I wouldn't be surprised if Pep employed a 3-4-3 as well (Pep always like to take risks after all). If the opponent doesn't get the ball, they can't harm you either?