I would lean toward the SQ2 over the FX. It is slightly more advanced, as it is a newer model than the fx which has been around for about 10 years without many improvements. The SQ2 already has a rest and sight, maybe not exactly what you want, but you can change them out after a couple of months if you want different ones. The SQ2 will probably have slightly higher trade-in/resale value than the fx in a couple of years even though the SQ2 is already a couple years old.
However,with all of this in mind, you have to shoot them both and then decide, you won't go wrong with the used bow though.