I am potentially in a similar situation (thinking about a 5x2x2)
Firstly im gob smacked the 3ft tall is the same price... make sure the glass is good and thick. would have thought it would need to be 15mm + for that height.
But anyway, back to the filter...
I just found out Zooplus are doing some cracking deals on the '06 range, and based on their prices, two 306's would be less than 1 FX5. But actually, for your 5x3x2, i think you would need 3 306's (or 2 406s)...
does anyone know if there are plans for a FX6? (I assume the 5 came out with the '05 range, but thats a guess)