Kind of a loaded question. MadMatt is right about the Taller 20" tire rolling thru bumps and chatter better. You don't want to tall like older stock tires though because of sidewall rollover and all, if your doing M-X. Desert racers seem to use taller tires because of rocks as well as X-C racers. I guess the style depends on which type of terrain you're on. Hardpack clay, loamy topsoil, Sand? It all would matter to the type of dirt you mostly run in.
Holeshots are great tires. Had the 19" M-X's on the Cannondale in 2001, they worked good. I don't think the 20's were out then. Now they have M-XR6's and S-X's. The 6's are supposed to be for looser terrain and the SX's are for Hardpack and only come in 20" I think. Have not tried Kenda's ,Maxxis,or BKT tires,yet.
Holeshots offer the 2 different types to suit your terrain. The others offer just size difference.