You need a big compressor to run an air grinder. The hose can be a nuisance. You want a grinder you can get two hands around - these are not common features of an air grinder. Speed adjustment on air tools is controlled by air volume.

The downside to electric grinders is they get so damn hot you need a welders glove to hold them. Cheap grinders will go through bearings quickly - this seems to be their weakest point. They're also considerably heavier than an air grinder.