I agree with SW03ES. The FX is a much sportier vehicle and the suspension is designed for this mission. I have driven an FX45 and the ride is VERY firm compared to the Rex. My wife drives our 2001 RX and she is very happy with the ride quality.
I drive an Infiniti G35 and the chassis is the same on the FX If you want a sport sedan with a huge trunk, get the FX. If you want a comfortable ride with more room and an nicer (IMHO) interior, get the Rex.