I know that the PS2 only fits in the Evo tanks, which it was made for. I don't have any first hand experience with either, but if you Google reviews of each, you'll find they stink, especially the PS2. I don't think Fluval has a clue when it comes to saltwater.
I have the Fluval Evo 13.5 for a QT tank and don't use a skimmer because the PS2 was rated so poorly. I use an air bubbler. But that's QT.
If you're looking for a nano skimmer, there are better options. How big is the tank?