When it comes to most cutting I like electric. A good 4" grinder you can do lots with and many attachments if you want a bigger disc but you'll need one with a removable guard.
Air grinders take significant volume CFMs to perform well and maintain power.
I have both and like my two handled air grinder for jobs that require a variable trigger speed. It is lighter in weight for lots of surface grinding and metal finishing. Most of the attachments; brushes, discs etc will work on both. Buffing is good with the air grinder, a flexible backing disc and bonnet.
You'll also want a small die grinder and air is fine. You can put on small discs and bits for detail work. Again variable speed.
