I think I've expressed my hatred for the XP3 on past threads... I'll leave it at that. I'm not a huge fan of any canister anymore to be honest, but I have an FX5 and an Eheim 2217 currently.
I'm a big fan of clear canisters which you can tell if it's time to clean the content. Fluval canisters are not clear, hence one of few reasons why I have none in my stand. For canisters, how can it be easier to operate and maintain rena xp's? I have personal experience with an xp3 that I bought used three years ago and still going strong today. I never own fluval filters, but have heard of complaints about maintaining them, except for fx5. All filters have strength and weaknesses when being compared against each other. Pick your most important requirements for comparison criteria.
I have 2 FX5s and I like them a lot. One is on my turtle tank so it has some serious work to do and it handles it fine. The purge outlet on the bottom is really nice too.