I ran two 220s with only one FX5 each at the shop I was running. One housed 2-20" pacus and an 18" Achara catfish, the other was a well stocked African cichlid tank. Extra flow for such a large, 6' tank is always good, but as far as actual filtration the FX5 should have enough. You may decide to add a second later, but I wouldn't assume you have to get two.
On that size tank I would look into a small pond/sand filter. The backwash option is amazine. You may go a little over your $350 budget, but it would be more than worth it. The ease of use and capability of such a filter is ideal.