Porto certainly benefited in the first leg of the round of 16 because they played in front of their own supporters, of course this provided more motivation in the match. I look at Porto, they really don't want to meet Arsenal in the round of 16, but this has happened, they have to accept it with full preparation, because playing against an English team is always difficult for Porto. However, Porto has experience in the UCL competition, Porto also has history in the UCL, and Porto is of course happy to be the underdog team in this match because this makes them have more strength and motivation than usual.
So you could say that in this second match Arsenal has quality but Porto has experience, on the other hand this will also be two great matches, Pepe will teach Saliba and Gabriel a lesson. It's true as you said, if Porto doesn't make mistakes in this match, I think Porto will win the first leg of the last 16 of the UCL league.
In this case Porto are only favored because they play home but statistically and probability-wise, Arsenal still remain above them.
Currently Arsenal's performance in domestic competitions, especially for the month of December where the matches are very dense there they are very troubled to make a breakthrough which in the end this makes their situation in the standings erratic but for the match against Porto until now it is indeed for Porto's odds a little superior but giving odds of 3.xx to Arsenal actually in my opinion this can be utilized well even though the match is still very long and we still have to continue to wait for updates on their development but I don't think it will be a problem if you try early especially for the odds at this time it is worth betting.