want to go brushless with my e-maxx

one of the reasons i went to the spool was cause i kept tearing right through the rear diff. what do u think i should put in as a diff? what kind of performance will i get with the Neu 1515 and the Quark 125? also i noticed theres 3 diff Neu 1515's each with a diff kv rating which is the amout of power im guessing. which one do u reccomend?