For a larger angle grinder, electric is the only way to go. I've used the crap out of my 4.5" Dewalt one. IMO, unless you have a very large industrial air compressor for any constant duty tools like grinders, electric is always better so you're not sitting there waiting for the air compressor to pump up or having to listen to it run all the time.
That being said a pneumatic right angle die grinder is also very useful for getting in tight areas that a larger grinder can't get in to.
Travis
72 Cuda