Tricky one, I'd agree with Josh as well. Rock 1 is the most distinctive, but really the sand and rocks should have the same or a similar colour. Thing is, when you do that, adding the rock is kinda redundant, most people will barely notice this stuff. Know what I mean? - if you go for say, Rock 2 you'll have a desert texture, if you have no rocks, you'll have a desert texture, but if you go for Rock 1, you'll have a desert texture with grey rocks around the place. Mixing the 2 rocks (say 1 and 2) makes sense, they'll accent eachother and justify eachother.
Are you considering rock terrain as well? - like make a massive rock, and place it just above the terrain to give rocky outcrops, or even just texturing the terrain with rocky parts. Just thinking that Farcry2 made good use of just a few cliffs and rock meshes, over a pretty massive map.
Health, Ammo, and bacon and eggs!