I think it will be 2-1 to Manchester United, a close game which could go either way. Daniel Sturridge to score a poacher's effort for Chelsea, whilst Wayne Rooney and Nani to score for United. I think both teams will get many chances, and the game will be open and players will be prone to mistakes.
United's attack has been brilliant, Chelsea have been consistent and hard-working although not dominating their opposition every game. However, I don't think both teams' defence is very solid, so I'm predicting some poor defending in this game. Both teams will rise to the challenge, especially Andre Villas-Boas who will be hoping to get a early 'big win' against rivals United.
However, to be honest, I don't see it happening. United have too much quality and are on unstoppable form at the moment, whilst Chelsea look less impressive than last season - they conceded against Norwich and West Brom and have been winning on very tight margins.
I think if Chelsea want to win the Champions League they will need further investment (which will then need to be spent cleverly) and more youth in their team, although it is younger than last season's.
United have been on top, top form but I'm not sure if Smalling & co. will handle the pressure of a big game like this, as they are still very young and improving. If Chelsea manage to win this one, I think United will bounce back and continue their amazing form from next game onwards.
The absence of Vidic (as stated by Sunil above) will be a massive blow against Chelsea, he is a fantastic leader and a no-nonsense defender against world-class players like Fernando Torres and Didier Drogba (who is injured, luckily for United!). However, I think their superior form (although not in the Champions League so far) and home support will see them through in a fairly open and interesting game.
2-1 to United, with Sturridge to score first for Chelsea, Nani to equalise and Rooney to score fairly late on and win the match for United. Looking forward to watching this, though it'll probably be on Match of the Day rather than live.