Bayern Munich doesn't have strong defense and it is same with Barcelona but these two clubs have good attackers. However, I see Barcelona having advantage of Bayern in this match because they are playing in Nou Camp and Barcelona have sharp attackers and quality finisher than Bayern. Lewandowski have no choice than to play against his former team. It will be his pleasure to score against Bayern.
It is known that Bayern Munich are always winning Barcelona whenever they clash for quite some times now. But let's not use the past to judge the future because things have changed presently and there is no assurance that history will repeat itself again.
Both Bayern Munich and Barcelona have weak defense, but just as you said, they have sharp attackers. The only difference i am seen in the two teams is the maturity of their players, if not they both have good players and the match will be an interesting one because Bayern will want to keep their winning record against Barcelona, and due to the fact that Bayern lost to Aston Villa, they will do their best to get the three points, which will be difficult because Barcelona also have good squad that can defeat Bayern Munich as well.
While placing bet on this game, I will bet the game over 3.5 because I trust all their attackers and we can see plenty goals in the game.