Playing United is becoming a foregone conclusion these days, so it would just be another match we would be favourites to win, and would more than likely do so too.
Playing and beating Barca is and always will be more important now, because it is that level of club we are competing for the big trophy - and recognition. Put it this way, those the world over will remember us beating Barca over two legs, thus progressing to the last 8 of the CL than they will us beating united in the least important of the English domestic cups.
Personally I think our name is on that Capital One Cup and as great as it will be to notch up another trophy, regardless of who it is we beat to win it, playing and beating Barca would dwarf it all day long.