I believe more in Roma in this match, where even though Roma has a not so good record when facing Porto, and this will be an away match for AS Roma, but currently Roma is slowly returning to its best form, in the domestic league they are able to make changes that make their position better.
Meanwhile, currently in the domestic league, Porto is gasping for breath to be able to maintain its position in 3rd position, lately Porto has experienced a series of bad results.
However, this is just an assumption, and we will never know for sure how the match will go, before the match really ends.
With the current state of form things are giving good advantage to AS Roma which have good boost after arrival of Ranieri who bring stability in this big club who was in deep trouble before his arrival while their performance and record in Europa League is always impressive. Porto is surely good club, but recently they are losing their rhythm and having no consistency which is the big problem for them in last eight games they have only one win with four loses which is surely big setback for them just because of this Porto is looking not good for having promotion.
First leg match is going to be important for both teams because if Porto fail to have any good advantage then surely it's going to be trouble while Roma will be surely needed to keep nerve and have good defensive strategy which will help them for having good result in second leg game.