I would be happy with the eheim on a 90gallon/ 350 litre tank. I would be looking for something that holds 3 to 4kg of biomedia in addition to any sponges and the eheim will do that. Flow rates on eheim filters are lower than other like for like filters, so it might not be great at providing mechanical filtration, but i would be happy that it could provide great biological filtration.
An FX6 is a bit of an overkill.
If it where me, i would use a pair of fluval 307s. Going with 2 would give you some redundancy should 1 fail, 1 at each end of would give you good flow around the tank, and each could hold 2kg of biomedia. If you prefered 1 canister, i might look at the new FX2 which fills the gap between the 407 and FX4.