Which motor?
 

I have an E-Maxx 12020/8XL with 51 tooth strobe slipper and stock diffs/tires using 14 cells. The 8XL is dead, and the 7 and 9 XL's are available. From my basic understanding the speed is the the 7 and the torque is in the 9. What would be the advantages of the two. I would like a little speed, but I do not want to overheat the controller. What pinion with which motor? I have 14,16,18,20. Thanks from a rookie!

Given the higher amp handling abilty of the esc you have, it should be fine to run with the 7xl so long as your truck isnt a 12lb tank. Whatever gearing you were running with the 8xl (assuming it was 'good'), then I would suggest to go down a tooth on the pinion to maintain a similar speed/temp with the 7Xl motor.