For ease of use, get two--one with a flex hose and pistol grip; the other with the rigid tube and lever handle.
Don't buy cheap grease, either, or you'll have oil leaking out of the grease guns. Mobil 1 synthetic grease doesn't leak but it costs about 4x as much as cheap grease.
edit to add: Don't forget, not all zerks take the same kind of grease, so you may end up with more that two grease guns.