You can also go for the more simple idea of having the wet/dry filter above the tank which makes the plumbing much more simple. Even under the tank setups aren't too difficult with a syphon overflow, etc...
With the FBF I imagine you would want a prefilter of some sort, but otherwise they are maintainence free - only really a problem if the power goes out I guess...
I don't think you 'need' a wet/dry, but for me it's the cheapest and easiest way to go. In my part of the world they charge up to $200 at the lfs for an AC500 And canisters are $200-$300... So a homemade wet/dry looked really good
W/D above the tank, like they did back then ... theres nowhere on my wall to put it, and its unsightly. The XP2, XP3 and AC500 should get most of the junk, and i can always prefilter the intake to the FBF and power here is very reliable.
Prices there suck, eh. I paid $50 for mine.