The FRX is the stiffest binding Flow makes, and would work just fine for you. With that being said, I'm a bigger guy and ride the AT, FSE, and FRX without any issues. All the baseplates in the NXT series are aluminum, with the FSE having a CNC milled base, making it lighter than the other NXT models. The FRX also shares the same highback as the FSE, just the baseplate on the FRX is not CNC milled, and the FSE uses the I Strap (More freestyle) and the FRX uses the A Strap (All mountain)
Either way, you can't really go wrong with the FSE, ATSE, or FRX.