If it says its sized for a 20 gallon, you should really believe its only good for a 10 gallon. I would get something better. A HOB fitler would probably be better than that.
Difference with the xp2 compared to xp1....Unless you plan on going to a large tank, an XP1 would be great. It's what I use. I just hate the pipes...better than green I guess. =p