In fact, these two clubs have almost similar performance if we both pay attention to the performance of both clubs and both clubs will both be superior when competing at their respective home grounds, such as Porto can win in the 1st leg and of course in the 2nd leg Arsenal far superior.
Bookies will still favor Arsenal in this match because we can also see that in the 1st leg Arsenal did not concede more than 1 score and that means Arsenal has great strength to be stronger at home.
Unless something happens that is unexpected, such as Arsenal's main striker or Arsenal's main defender getting injured because we don't know what will happen because there are still 2 weeks remaining and in the remaining time a lot can happen but I hope Arsenal will be fine and can proceed to the next round.
Losing the first leg in extra minute is very disappointed for Arsenal. If it ends as a draw, Arsenal will have easier task in a second leg and it shows their players still lack of focus on important time that can decisive a match and even an aggregated result.
For the second leg, Arsenal must beat Porto but they will have to focus on their defensive system, tighten it to avoid a conceded goal at home. If they let Porto scores even a single goal, their task will be much heavier and they will possibly face with a nightmare at home too.
Porto are very experience in Champions League and they will be able to create a lot of challenge against Arsenal then wait for Arsenal's mistakes, suicides. Last season in Europa League, they lost at home even had a very impressive performance in a first leg, home away.