Just a little note about my experience...I have worked with a Fluval 405 and a Rena XP4, and flow rates aside, the Rena is a much better designed unit in my opinion. Opening and closing the Fluval was a fight each time, and filling and starting it was not a very smooth process.
The Rena is very easy to work with, opening and closing it is easy and simple - the way it should be. The procedure for filling and starting the Rena XP was also a much easier and smoother process. It just works perfectly, no priming needed.
I'm not saying that this is absolutely going to be the case for everyone, I'm sure many people are happy with their Fluval 205, 305, 405, etc, but I will buy a Rena XP series as my next external filter.