Good air tools are expensive and cheap ones don't last long. They also
require lots of air to run properly, causing your compressor to run a lot.
Think about what it costs to run that 5 hp compressor (and the wear and tear
on it) vs. the motor on an electric angle grinder. Go with electric.
FWIW, I have another slant on how expensive of an electric one to buy.....
Four or five years ago, I bought two elcheapo Harbor Freight 4" angle
grinders. I keep one in the basement shop and one in the garage. Each one
gets about half the use and at this point, they both work like new. I have
less invested than in one expensive one and I always have one where I need
it when I need it.