Arsenal still got a chance to play and get pass this stage of the champions league and also at the same time they are at the verge of loosing it to the Porto too because if they allow their self not to in the second leg, they may stand the risk of not been able to continue with the champions league again this season and it will be a good achievement for Porto because I'm sure they wouldn't believe they would make it to this point.
There are many possibilities and scenarios that could happen in the second leg, you can see in the picture below, Arsenal is the favorite with only @1,24 odds. But we know that Porto have an advantage on the aggregate side which makes them more confident in playing defensively.
Well, with all of these and as I said before, I wouldn't bet on this match because the risk is too high.
This first leg makes the whole duel here pretty exciting as you said. It opens up a variety of options for Porto during the game and they are not the worst at playing counter attacks. I think it is still somewhat hard to believe that Arsenal would give this away at home, but it is still an advantage for Porto and imagine they play one successful counter attack early on in the game and they extend their lead by another goal. That's the moment when Arsenal would realize that the next round is at serious risk.
But Arsenal in great shape. I think the odds for Porto could be interesting for someone who wants to take a massive gamble and bet on Arsenal messing this up. It is an uncomfortable situation for Arsenal, but they can be lucky that it is not the rule from the past where goals away counted double.