Limited experience with sumps here but that probably would be the way to go. FX5 is a good filter but I am truly starting to believe its all about media volume.You need a decent flow rate, but from all the reading I have done its all about surface. Boy am I going to get flamed for this, Allot of the Oscar folks (many have huge tanks) like the Aquatop,Marineland 530,Rena XP4. These models are massive they hold ALLOT of media, 22" tall with four baskets. They are much cheaper, 3 year warranty, excellent customer service. And I agree with the Oscar folks when they say I think very underrated? Lets face it , Oscars can keep right up there when it comes to Bioload and Mess.
I own a FX5 also and side by side the Marineland (or many other brands) holds more media and gives you many more choices of what you want to use for media. My only issue with the FX5 is that its all sponge and dosn't have near the space for say Biomax, or Matrix that you can get in many other filters. Heck with these filters you can fill the entire filter with just Biomax (and with all four trays thats allot) and still have money left for another one to fill with all mechnical media.
So, Sump is the Pro way to go. Unless you are one of the lucky ones with plenty of $$$ to blow just wanted to throw this out there. You can buy two (or almost) for the cost of the FX5 and Eheim...Well. Just check the oscar sites quick and look at the specs one for one? And when you think about it is the fish going to give a ???? if its a sump,Eheim or a FX5 as long as its getting the job done? And yes I still want a Eheim but?