These are two things that should be used together, not compared. One is a deterrent, one is your last ditch defense. These should be layered with the forethought to try to avoid startling the bear in the first place.
-john
I am firmly on the fence about this. I hope never to be in that situation but it's my understanding that these attacks happen very quickly. I wonder if a layed, 2 step approach (spray and then gun if the bear keeps coming) is realistic? I'd probably want a gun if I was up there, but I'd definitely have bear spray on my belt.