I would have to give the advantage to Milan for these reasons:
Milan is very much like Barcelona in terms of controlling the match. With players like Pirlo, Seedorf, and Kaka, they can have much of the posession and control the match, sort of like Barca, minus all the players caught up in attack. I know Chelsea does not need that much posession to excel, however there's more.
The difference between Barca and Milan is that the latter is never exposed at the back and usually has players behind the ball. Barca does not have a stacked defense, as Puyol, Oleguer, and Belletti do not match up with Nesta, Stam, and Cafu. The individual talent of Milan's defense is far superior to Barca. Milan is also more organized than the Catalans as well.
Now, since Milan is rarely caught out at the back Chelsea will have a tougher time striking on the counter attack, something the latter does very well. Chelsea's first three goals against Barca were all on the counter-attack, some of which was due to errors by Barca defenders. Milan will not be as loose at the back as Barca, so Chelsea will have a difficult time scoring IMO.
Milan can hold posession and have the players to make you pay, especially when Shevchenko comes back into the fold. I just do not think Chelsea will have an easy time scoring against Milan, and if the latter sits back like they do on the one-goal lead Chelsea will be in trouble. The latter's attack is no better than Man United's for that matter, however Milan is strong in all positions and know how to control a match.