I have a diamond goby that eats prepared food well and is also always going through my sand and turning it over. If they are eating well at the store, it's probably a good bet. Gobies that are sand sifters include the yellow headed sleeper goby, the diamond goby, the tiger wardi goby and a few others of course. The one's I named are what I usually see at my LFS.
Conch's are good too, but I would be concerned about running into the same issue that an earlier poster mentioned about the sand sifting gobies. The conch will probably go through the sand bed pretty quickly and then will need to be supplemented with other food. I think gobies are easier to supplement than snails. This is of course unless your tank is huge, in which case they probably wouldn't depelte the sand bed of food. Both animals do the job well, but will likely need to be supplemented with other foods.