United are probably going to win
You get the feeling that, for all their qualities, Barca are a plan A kind of team. They play in one fashion and rely on individual talent to succeed. United on the other hand are a team of many faces and a team of many tactical plans. Fergo has truly become a tactical mastermind, and you really can't see him failing to work out a method to shut down Barca's plan.
I suppose it all comes down to team selection - Barca will choose one fairly predictable team, with perhaps a few leftfield choices due to unavailability. I think Fergie might be inclined to play 4-4-2 to add some midfield wing play and try to shut down Barca's WFs. Evra vs Messi may well struggle, Evra and Park vs Messi could probably handle him. On the left you might have the issues though - John O'Shea is bobbins and may well struggle to cope with Terry, especially with the scant cover in front of him. Either you play Ronaldo on the right and get him to track back, limiting his forward effectiveness, or you move Park across and give Messi more of a free reign. On the other hand, perhaps the bare minimum of cover will work on the wings, as long as things are tight in the middle - Barca are hardly a crossing team, and try to work things throught the middle whenever possible. You have to wonder about the centre then.. Carrick is a definite, but who do you play next to him? Giggs is too slow for their speed of passing, and Anderson is a little bit gash. I think Utd could feel the absence of the workhorse Fletcher, since controlling the Barca mid three is going to be very tough work.
Up front is a tough choice. Rooney is a definite, but i reckon Berbs could cause some serious problems for Barca's back line as well - this is the sort of occasion where a cool head is needed, and Berbs' near sociopathic level of emotional disinterest could really profit, especially against a defence that might give him slightly more space. I can very easily see him playing the ball into Rooney or Ronaldo for a United goal.
In terms of the actual tactical stance.. well, i don't think United are going to be attacking very much at all. Ferguson has too much of a pragmatic attitude to go all out, he knows that if you can block Barca and attack on the break (in a slightly less twatty style to Chelsea) then you can get something. Both teams are very used to having a lot of the ball though, and when it comes to keeping possession Barca are near unbeatable - perhaps Utd could find chasing the ball all the time to be far too much work to handle. Take a look at the last game against Arsenal, where they constantly gave away the ball and spent the entire match running after it. There is the possibility that the same could occur tomorrow - a skillfull, possession orientated team making Utd work too hard and breaking them down. This season United have relied on strong, decisive midfield play with an emphasis on ball retention - when they have to chase the game i'm not sure if they'll have the calmness of mind to succeed.
Well, my opinion on who's going to win has swapped over half a dozen times in the time it's taken me to write this. But you can't escape the nagging feeling that Taggart will have Barca sussed out in a tactical sense, and it will take some supreme individual skill to outsmart the red fuckers. But hey, we're talking about Barcelona here!
If i was a betting man, i would put some cash on a 1-1 draw and some cash on Utd to win on penalties - you can easily see Utd scoring, and you can easily see Barca getting back level with a moment of sublime skill. And you can see them both hanging on for extra time in that situation. But apparently there hasn't been a goal in extra time in a Champ Lge final for 25 years. And look! Barca haven't participated in a competitive shootout for a decade, and a European shootout for 20 years. The writing's on the wall folks, go put the farm on it.