Depending on how the sump is setup the FX5 may be better at mechanical and biological. I don't use sumps on freshwater, the FX5 is just a better option either way IME.
I find that properly set up sumps are far superior to any canisters setup anyway. The advantages ( including cost ) have always been with sumps IME. You would need several canisters to equal the filtration abilities of a good sump. Plus, it takes about 15 seconds to throw a new sock on a sump setup. How long does it take to do maintenance on an fx5?
That being said, if I were to run both, I'd most likely setup both bio and mech in both for redundancy.
My tank Overflows through a Filter Sock into a 38G tank. I breed Cons in the 38 Gallon Tank. I have an FX5 hooked up to the 38G and pumping back into the main tank. As well as a Mag 12.5 pumping back into the main tank.
FX5 is set up with standard sponges, and baskets are filled with Matrix. On the top of the top basket I have a polishing pad the size of the whole top and it's cut so that is doesn't black the pipes. I opened the canister 2 in the last couple of months. 1 to replace the pad, and 1 just to see what it looked like inside.
My water is always 0 0 0, Tank is a 150G and stock is 2 Os, 11 Giant Danios, 1 6" Pleco, couple large snails, and about 50 1/2" Convicts. Gravel vac 1x weekly, and 30% water change 1x weekly.