Barcelona's defense looks better and in these matches it will be important how well MU attack will play. Barcelona will most likely be able to score, by mid-February Lewandowski should be back in the game, this is very important for them. The MU defense is not that strong, they will focus on the attack. In fact, the match is very difficult, perhaps even worthy of the semi-finals.
I think United offense could be a little weak right now, but their defense is quite good, whereas Barcelona has a great offense but a weaker defense, even though it has been better, still not great. This tells me, maybe these games will be filled with plenty of goals? Like a 4-2 type of score doesn't sound unlikely to me, it sounds quite like how it will probably happen.
I am still believing Barcelona to beat United in these games, but even if they don't, I am expecting a lot of goals. United got a lot better compared to start of the season, and Barcelona has been pretty good all year but somehow played badly at UCL and that's why they are here.