Most likely in the first leg, Roma strategy will focus more on the defense line, at least being able to bring home one point from Porto is a good result, although it will be quite difficult because playing in front of the opposing team supporters will at least affect the players mentality. On the other hand, of course Porto doesn't want that to happen, so winning at home will be the price they have to pay. In terms of chances, I see both teams have an equally big chance of qualifying for R16, but it also depends on how prepared and serious they are in playing these two legs.
First leg is going to be important to Porto is having not good time while AS Roma is also facing in away games but if they are able to hold things on level then surely they could be able to done good performance in second leg which is going to play in Roma where they can do as they wanted. Porto and Roma both are same level clubs, but recently Roma have good experience and quality in Europa League while Porto is struggling in all leagues which is concern for them but in this game both need to hold nerve and also give their best which help them for having good performance.
Roma strategy could be played with tight defense because second leg is surely into their hand where they can give good performance and also have good chance of going through into next round.