This is a question I've been pondering myself as I'm considering doing a 125gal down the road. In this case the FX-5 (up to 400gal) would have more filtration capacity than the two 406's (100gal each). In my case I'd go with a single FX-5.
I suppose you could do two 406's as one would serve as a backup for the other. However, if cost isn't an issue, I'd see no reason why you couldn't go with an FX-5 and a 406 (your backup if the Fx-5 dies).
75g Planted Barbs Denison, BlackRuby, Tiger, Albino Tiger, Green Tiger, Cherry, Gold, Checkered, Odessa.
NonBarbs Cardinal, Neon, RummyNose, Bloodfin Tetras, Z. Danios. 1RTS. Amano, Bamboo, Ghost & RCS. 20g Long Shrimp Tank RCS, Tiger, Amano.