Im ganna shoot with Rena. I have a X4 and it is great. I clean the canister once every 4-5 months as I hear Fluval you have to do it more often cause it sucks up so much. I also like the accesories that Rena give you to be able to change it up a bit. And their customer service is AWSOME!!!
I never owned a X5 but have been tempted to get one. Only thing is it takes up to much room in comparison to the Rena. Not bashing the X5 for 1 sec, just feel the Rena fits me and my set up more
I also like the flow of the Rena as I have a U/V connected to it. I say the slower the current the more the U/V hits...again, JMO