Mach.
Amazing speed>Bunny hops
The Mach back also makes Egg hatching much faster, it gives you the Route 110 mini-game, you must use it to catch Rayquaza, etc. How can't anyone love this.
Mach, because like other people said, its faster and good for hatching eggs. Plus I like fast things. Acro bike is way to slow for me; Mach all the way.
Acro bike for me. I still haven't practiced enough with the Mach bike so I still can't control it well. I've only used the Mach to get past crumbling floors, get up slopes, and take the Mach bike challenge on Cycling Road. The rest of the time, I always have the Acro. I think it's fun hopping from side to side, doing wheelies, and doing bunny hops.
Acro bike for me, because of the fact that it can do tricks, is moderately faster than running and can do cool tricks. And is usefull for places that have those pebble things.
Edit: but i also like the Mach bike for its speed. Though it is a real pain to drive.