Between Napoli and Barcelona currently, there's no much difference because both teams have flaws they're battling with. For Barcelona, they're not so grounded yet as there's a bit of shakiness and inconsistencies in their usual performances before now. They're now at a low leveled performance phase than they are normally known for.
You're right. The performance of Napoli and Barcelona seem not so different, they are no longer playing as well as the last season. So, it is quite difficult to predict which team to win the match. If we look at the quality of attackers, Napoli seems better than Barcelona. Barcelona has no productive attackers, Lewandowski can't play as expected until now. Napoli has some attackers who have better productivity in scoring goals (Victor Osimhen, Kvaratskhelia, and Raspadori).
Fixture between Inter Milan and Atletico Madrid gone be a blast. This is the same Inter Milan that got to the finals last season, dropping out so early? They've done that too much. Last season they scaled though. I be expecting same thing from them in this round.
Inter Milan was the finalist of UCL in the last season. I'm sure they will try all out to reach the final round again for this season. Their performance looks quite good so far, they are unbeatable during the group stage matches. Against Atletico Madrid, I believe Inter Milan has something better than Atletico Madrid. So, they won't be eliminated in this round, I guess they will beat Atletico Madrid.