In my opinion RENA all day!!!
My first tank ever had a xp1 on it(56g) and it keep the water crispy and was very easy to clean. Once I up graded to a 90g I brought another xp1, with both of them on the 90g it also kept the water crispy but I also felt that I had to do more filter(s) cleaning. So once I brought a 55g I took the 2 xp1's and put them on it, keeps it SUPER crispy. Now my 90 is ran by one fx5 and I don't have any complates except the one time the motor was stalling(carbon had come out of the bag).
Now comparing the two companys. I'll say the fx5 is one hell of a filter but Rena's are easier to clean and are also very good filters!!