Porto is really popular with their defense skill, even they lose, they won't let the opponent to score more than 2 goals against them.
You don't expect Arsenal to win? Well, then you are onto something I guess. The odds are pretty good if you want to go with Porto.
But what is the point in your comparison? Porto is playing the worst season in Primeira Liga since a very long time. All the last years they have been far better at this point than they are now and you call the results "outstanding". I am puzzled to be honest.
Arsenal on the contrary is leading the Premier League ahead of Manchester City and Liverpool. I think that says it all. If we consider all these facts, I am pretty sure that Arsenal still has the upper hand. It doesn't mean things can't go wrong, things can go wrong of course. But Porto is out of shape and got a lucky win against Arsenal in the first leg. Let's see how they will play away from home this time.