I think this will be a very good and uncertain match.
Both teams were very good in the first round of the group, but Bayern had a much stronger opponent, and at the same time showed great quality and dominance.
I think that Bayern found a very good replacement for Lewandowski in Mane, and they dominate again in the Champions League and in the German Championship and I give them a slight advantage in this match.
Barcelona also brought in new players and raised their level of play, and I believe that there is no chance that they will be eliminated in the group stage of the Champions League this season, as they were last season, although they are in a very difficult group of the Champions League.
Bayern is a lot more established club with most of their players, even though they lost Lewandowski, they got Mane instead and he is a great player as well, on top of that they also have players that could in return play very well and been playing together for a very long time as well.
Barcelona on the other hand got new players on all positions and there are only a few players coming back from last year, they do not have the chemistry that Bayern has, and neither way more talented than them neither. I am guessing Bayern will probably just run hard and tire out Barcelona and beat them by 70th or 75th minute, and score easier after that.