Aluminum oxide lasts a long time and isn't too coarse when new, takes off rust great but eats up nozzles in the process. Not cheap, but a little goes a long way.
It really depends on what you are blasting, there really isn't a single media that will cut rust aggressively and not chew up soft metals like aluminum, you'll either have to use fresh glass on the ugly stuff and blast the progressively softer stuff as it wears, or save up batches of parts and swap medias to suit them.