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.
For Napoli, the team just got a new boss and so Mazarri is trying hard to implement strategies that best fits his team and would give them the required results. There are still weeks to the round of 16 games, so it's more chance given to both teams to improve themselves in their various league games and then be better prepared for the encounter in February. Judging both teams, they've got same odds of qualification based on the current teams' technicality and definitely the players in the team are eager for a win.
As of 3 weeks ago before the round of 16 was drawn, Napoli was conquering teams in Seria A, it was one of their best recovery I have seen this season but I'm not sure if it was a coincidence or perhaps normal football thing but they have not impress me since then, I was even among the few in supports that their current form will smark ace Barcelona with their feast but I'm beginning to change my support on that, just to be at a safer side a single match bet might give high odds to bet but not a good option because Napoli is not as strong to beat Barcelona without going through a lot.
Napoli barely even keep to their domestic league, I'm skeptical where they intend to bring that level of form to match up with Barcelona, they are not up to standard in my record but who knows what will happen before now and February. Between now and February 21 of next year has about 6 weeks or even more and it's possible that within this space, they will improve their ways and possibly come to atleast 5th or among the top 4 and then I will reconsider my opinion about their future of the Champions League.