I am trying to decide between the 19x6 and the 20x6 Holeshot MX fronts. I only do mx so ground clearance is not a concern.

 

Big jumper - go to the 20's
Small jumper - go with the 19's.

The 19's on the top MXers were breaking shocks and bending rims, so they made the 20. The 19 has less sidewall flex, so handles better when on the ground. It has less clearence on a rutted track. It has less rolling resistance or rotational inertion, and is lighter. Unless you are doing big jumps, go with the 19's.