Hmm.. Shoulda put a poll on this thread..Responses all over the place.
That is due to the fact that the answer is based on what outcome is desired. Typically in a residential house, flat is used on the ceilings for the reasons mentioned. It hides ceiling imperfections, it cheaper and getting dirty is not a big concern.
In a shop environment, needs change. Usually one desires more of a well lit work area, the air gets dirtier and the beautiful looks of a nice flat ceiling is not a concern. Flat paint is very chalky and hard to clean so some prefer semi-gloss or eggshell since it reflects more light and is easier to clean though I know of nobody that wipes down the ceilings of their garage.
Then you need to figure the purpose of your shop? Is it too do sandblasting and engine rebuilds or is it nothing more than a showcase for your cars and tool collection? As far as spraying or rolling, that varies too. Is the room empty with very few windows, doors and cabinets to mask off? If so then spraying in my opinion would be easier and look better. No brush or roller marks. But paint will get everywhere so if your shop is filled with tools and stuff, rolling or brushing might be less work since you do not need to remove everything from the room and mask off doors and windows and cabinets.