I have a 120g (60x18x26) and use two Cascade 1500's on it. For the price I paid for the two of these I could have got a single FX5. The only problem I had was that my stand would not accommodate an FX5, there was no room. I know that the stock output elbow would create more of a current than I would like so I would make a custom spray bar, or do the PVC elbow mod that some do on them. So personally I would totally want an FX5 on a 120g.
My latest project is a 210g (72x24x29) that is replacing my 120g. On this set up I am going with dual FX5's with custom spray bars to cut down on the current.