Barcelona's condition, which is not at its best, raises a little doubt that this team can survive longer in the Champions League. Barcelona may be able to beat Napoli who are slumping in Serie A, but will struggle when facing tougher opponents in the next round. I believe anything can happen in football, if you look at the performance of other teams such as Real Madrid, Bayern Munich and Manchester City it will be difficult for Barcelona to get rid of these teams.
Both teams are not in good condition at the moment so it will be quite difficult for both of them to get to the next round. But for the round of 16, perhaps Barcelona is far more favorite even though the first leg will take place at the Diego Armando Maradona stadium, but this does not provide a guarantee because Napoli also have the ambition to beat Barcelona in the first leg. So when they visit Camp Nou it will be a little easier and there won't be much pressure to play at Barcelona's home ground.
I wouldn't put these two teams as favorites because they are both in quite worrying transitions. Barcelona is experiencing a club financial crisis and Napoli is also undergoing a change of coach who so far have not found the same rhythm as when they won Serie A last season.