The main difference is that they get a fraction lighter and stiffer as you move from the Relay Pro to Thruster to XLT. Most people ride the Relay Pro great all around flex. The XLT is very popular too and it's the stiffest although I don't notice the difference unless I'm pounding through crud and moguls. Relay Pros work great in everything, a good balance. If you're in to miny tweeks and adjustments you can move up the line for a little more added stiffness. You would never get rid of the Pros that's why I just rotated the XLTs into one of my other setups.