I have a little experience with both of these things. The taser will immobilize your target, when I was tasered I pretty much just dropped to the floor, it didn't hurt but it was a very sudden and startling thing. I was able to get up after a few seconds though, but I think that was just an oddball thing, I'm quite sure you're supposed to stay 'tased' for a lot longer than that.
With the Pepper Spray, if you ever get a can of it, just fuck around with it and you'll see how ridiculously strong it is. I had a can once and I sprayed on the floor in some parking lot, it made my eyes water and made me want to vomit.
I'd go for the Pepper Spray, it's easily obtainable, and I feel like it would be a lot more reliable.