Grit is a nebulous term which can describe any small particle. However I think in the context you are describing it would most likely apply to silica sand.
Garnet for abrasive purposes is a crushed gemstone, Silica sand has a hardness on the Mohs scale of about 7 while Garnet is a little harder at 7.5 to 8.5 .
By its nature Garnet is more expensive than sand, and it is preferential that it be used in equipment with recycling capability where it can be used about 5 to 7 times. Sand is most often used as a single pass blasting operation. In practice Garnet will clean faster, but that has to be offset against its higher cost of about 0.31cents per pound versus about .09 cents per pound for sand .
B.E.

You are judged not by what you know, but by what you can do.