Im going to stand behind powdercoat. I really like it, its super tough the only real time your going to chip it is if you smash it into a rock or something. You can literally bang on it with wrench's and not have any ill effects. We ride in coal mostly here in PA and its pretty nasty stuff, ive seen it almost penetrate painted parts (they stay a tainted black color even when cleaned) and it can be difficult to clean painted items covered in coal dust but it wipes right off of p/c. Its not terribly expensive figure about $100 to have a frame done. I just got done prepping my frame to go to p/c and what i did was go to the local hardware store and bought cheap hardware to put in all the threaded holes in the frame (make sure you bottom all of them out so you will have good contact for any grounds you may need or you will have to grind the powdercoat off). I powdercoated alot of the small parts myself with a little spot blaster a craftsman powdercoat gun and an old oven that i bought off craigslist. The only thing i can say is a negative for the powdercoat is the fact that yes it is more expensive than paint and you have to remove every bearing/rubber bushing from the item your powdercoating because in the curing process it will melt)
I dont know what sand is like but i could imagine that riding behind someone (especially a banshee) would be like a low pressure sand blast which theoretically would deteriate paint, but like i said i have no experience with riding in sand. just my 2cents