I've seen a request to have someone from the company to jump into the discussion, so I'll give you some information that should help. I'm the marketing director for MOTIV (but don't hold that against me).
First, when we developed our hook potential rating, we had the ball dynamics in mind (RG,diff, etc.). So, with the same finish on the balls, the SX1 will be more aggressive because of the core design. It's not rocket science to know that you guys can tune the cover where you need it to be.
Out of the box, the SX1 finish is better for medium oil and the TX1 finish for medium-heavy. We are hearing the best results from bowlers that put a 1000 or 2000 abralon finish on the SX1.
Our reactive urethane Motivator-X cover is really durable and has a lower oil absorption rate. You won't get a massive jump on the backend, but a smoother reaction. The lower oil absorption rate also gives you extended ball reaction life and better consistency from game to game.
Scott Hewitt
Tech-Line Products (MOTIV)
http://www.motivbowling.com/