If you think the flow is too much, just add a spray bar, if needed drill extra holes in it and point it against the tank glass. The flow can also be reduces on the filter anyway.
I always go for the most flow and biggest filter possible, leaves more room for error. If you are only lightly stocking the tank, a smaller canister filter like the 406 with additional flow pumps would give you the best flow distribution and flexibility (you can always re-adjust the positions of the flow pumps).
If you plan on big fish, for heavily stocking with small ones, you'll want the extra mechanical filtration from the FX4.
Either option is fine, whats best depends on what fish you want to keep.