Two 400's are really all you need on a 100 gallon tank. A lot will depend on how you plan to stock the tank. Canisters can extend the time between filter cleanings but they also take a lot longer when you finally do servace them. What ever you choose...avoid mixing brands and stick with one or two types of filters on all your tanks.
We've found it a LOT easier to keep repair and servace parts on hand that way...and of course having a couple of spares around...just in case.