I'll look into that possibility later, but I think Manchester United could be the favorites at Old Trafford. About Manchester United, they are much better now and I'm happy with what they had at Camp Nou yesterday.
In how many league matches, if I'm not mistaken, Barcelona only conceded 7 goals. When against MU, only 1 match, Barcelona has conceded 2 goals. From that match, we can see that Barcelona's defense counter is not a 1-2 bait or a true pass. But just dribbling, Barcelona defenders tend to be solitary and compact, but if your opponent is dribbling, it looks like Barcelona's defense can be penetrated. I also believe in MU's second leg there is a greater chance of winning.
You know what? After impressive manchester united first leg to hold barcelona at home and have 2 away goals advantage that make them only need to get another draw of 0-0 or 1-1 while barcelona will try everything to win the game or at least score 2 goals or more.
This makes me even think to go for barcelona to win instead manchester united, they had great history in the past on coming back in the second leg.