The short answer
For most Arizona garages, a polyaspartic topcoat is the better choice. It is UV-stable, so it won't yellow when the garage door is open, it handles hot tires, and it cures fast enough for a one-day install.
Where epoxy still makes sense
Epoxy is a strong, affordable base coat and works well indoors away from sunlight. Many of the best systems use an epoxy or polyurea base with a polyaspartic topcoat to get the best of both.
What really decides how long it lasts
Surface prep. A coating is only as good as its bond to the concrete. Diamond grinding opens the concrete so the coating can grip; acid etching and DIY kits often peel within a few summers.