I would run the ubec. the 9920 has a bec but it don't work on high cell count. here is a site to
http://www.bk-electronics.com/ The xl is a longer can with longer armature to make more torque. I like the xl motor. Mike runs a 1940 in one of his trucks over the other motors. That should tell you something. I have been runnig a smaller one around to test out. I can see a difference in the lehner motors.