Barcelona wasn't in their good form, but it's a big joke if they're lose against Shakhtar Donetsk and Royal Antwerp because both of them are in different level. If Manchester United was in this group, I believe Manchester United will able to qualify for knockout stage.
Either Barcelona or Napoli win the match, their next match is against Arsenal which I think they would lose.
Are you sure that the winner of the Napoli - Barcelona matchup will have a draw against the winner of the Porto - Arsenal matchup? Because I can't find any information related to the quarter final draw now. Maybe we might have a brand new draw for that stage. But if this really happens then I also think Arsenal will eliminate Porto first and then do the same for their next opponent as well.
I'm saying this no matter which one is their opponent. Because Arsenal are currently more powerful than both of Napoli and Barcelona. But when it comes to making a comment on which one of them to advance to the quarter finals I'm closer to Napoli for now.