I see Arsenal been the favourite in this game as a trap by the bookies because looking at every Indication the probable chance of draw is way higher to a win, although Arsenal has been doing great in the premier league they still doesn't look too convincing for the champions league because the game in the champions league is actually very tensed and way too competitive than it is in various domestic leagues.

Porto since after their first encounter with Arsenal has continued to still remain firm in their performance and I think they will be able to put Arsenal on their toes by not giving them an easy game to win and looking at Arsenal last performance which they literally struggled  against Brentford, it's like an indication to the fact that their chances to win Porto isn't that great because normally previous games before the next has got a way of affecting players psychology and confidence too except Arsenal is quick enough to snap out of it they seem to be setting into a new slow down period.

A draw will actually favour Porto, even tho the gunners may try not to want to end their champions league journey at this point a draw wouldn't do much for them because they are already a g behind of which they will have to make up for if they must further proceed into the next stage of the game. Porto on the oder hand wouldn't want to jeopardize this opportunity so they will fight to get a draw which will still be in their advantage.