Fine. T64 FX and T64 have the same topsheet but T64FX has a softer sponge. The topsheet for T64 already feels relatively soft so this makes T64FX feel softer and have a nice dwell. It's arc is slightly lower than T64 and on drives, it seems to play faster with a slightly longer trajectory. However, it is easier to spin on low speed drives with T64FX, but on harder swings, T64 produces more spin.
Which you prefer is ultimately about your playing style. They both reward players who like to block and back up, maybe T64 FX rewards the player who plays further back more and T64 the player who plays a bit closer, but still further back. But if you swing really hard, use T64 over T64FX.