I just did mine and I used semi-gloss white for the walls and ceiling. I just like the shine, the light reflection and the clean, sealed look along with the possibility of actually being able to wash the walls if needed.
My wife is a big Tennessee fan and she painted a big orange T on one part of one of the walls along with a white and orange checker board above it but that is the only color. I wouldn't let her paint the T on the floor.
I would deifinitely do semigloss over flat. Flat is extremely difficult to clean and in a garage it stands a pretty good chance of getting dirty, so semigloss would be preferable. Also, I just like the way semigloss looks.
I suggest that you don't use texturing on the ceiling of the garage. The prior owners of our home used "cottage cheese" texturing on the ceiling and it is now falling off in about 4-inch diameter pieces.
I don't know if this is due to their applying it improperly or because of the humidity & temperature conditions of the garage.