Bot teams in this game are playing great, their main philosophy is to attack and score. However, even if an over 2.5 game is very possible, I think Barcelona will be glad with a draw as they also were against Stuttgart or Lyon and Bayern last season when they played away in first leg. They will have a good chance to win at Now Camp.
I think Arsenal will have a lot of scoring opportunities, I'm pretty sure that they will make a good game. It's normal for Barcelona to be favourite by the bookmakers, but I think Arsenal is good at home, and if Stuttgart managed a draw against the catalan, why wouldn't Fabregas and the company?
I can't predict what will happen in this game, and no one will do, but the best choice for me is a draw bet here.