9853/41001 = balls to the wall, plenty of all around power doesn't really sacrifice top end either

9853/41021 = smoother low end, probably a little better on a dusty low bite surface.

Ran both of these combos in truggy last year with the elite 5 and p5xlt, for buggy 9901 might work a little bit better but it probably depends which engine you plan on using also.