Disclaimer: I'm by no means an expert, what follows is simply my own opinion.

This (a cheap plastic squirt gun) probably wouldn't be as effective - you'd need a direct hit in the face and I'm not even sure that would be effective.

With an aerosol spray, you get a cloud that will go into the eyes and nose, causing sneezing and temporary blindness.

If it's for use on animals, I think I'd be more inclined to purchase commercial bear spray.

If you're being stalked by a predator (rare but it does happen) they're probably going to be approaching from downwind of you, in which case the wind will, if anything, carry the spray away from you; otoh, if you inadvertantly blunder into them and they attack because they've been startled, then they'll more likely be upwind of you (otherwise they'd have sensed you coming) and the wind will be working against you. Since most bear attacks are of the latter variety, I'd consider pepper spray to be kind of a last resort; a rousing chorus of "Barrett's Privateers" as you're hiking through the woods is probably a better deterrent <img src="/images/graemlins/smile.gif" alt="" />