Looking at the odds, Arsenal remains heavy favorite.
It's really worth to bet draw or Porto to win, looking at their previous match, Arsenal was struggling to beat Brentford while Porto is still strong until now. So I expect this match should ends in draw, Porto don't have to win while Arsenal will try their best to win.
Porto will try defensive tactics because they already have the advantage in the first leg. Porto can play very well with their defense. This can be seen from the first leg. But Arsenal played as host in this match. That will give them additional morale and motivation to win the match. Even though it was difficult to win the match against Brentford, I think the match against Porto will be different because Arteta will really prepare a strategy and Arsenal will go all out to win the match. I predict Arsenal will progress to the next round.
It would be an easy 90 minute win for Arsenal. Porto might have a lead over goal but they are no match to what Arsenal is now. The next game is going to be in a much different context as well. Arsenal has placed themselves as the contender for title of the premier league and the match is being hosted in Etihad stadium, the home ground of Arsenal. Arsenal will most probably have a better aggregate than porto during the gameplay and advance to the next round.
Another match on the same day Barcelona vs Napoli would be much competitive one but Barcelona would have a better play.