Porto vs Arsenal, I think in this match both clubs have the same chance of beating each other, with Arsenal having a bigger chance in my opinion. In the domestic league, the two clubs are equally strong and have not so much difference. However, considering that Arsenal is in a more competitive league, on that basis I see Arsenal as having a better chance of advancing to the next round. But at the same time Porto is also a club that can do something big and they are very likely to beat Arsenal in this match. Or it could be that the two clubs will beat each other in 2 matches and at the end they will calculate the aggregate goals, or even have to decide in the penalty shootout round.
Furthermore, I have to say that the remaining matches don't worry everyone because Dortmund, Inter Milan and Arsenal seem to have a big chance of qualifying, but Napoli against Barcelona is a match that is difficult to predict because these two favorite clubs are both experiencing bad times but both clubs have the same chance of qualifying for the next round but personally I would prefer if Barcelona would qualify for the next round because Barcelona has a lot of experience in the champions league even though they will only rely on existing players but Xavi is able to give his best.