Bear spray has come a long way in even the last 10 years. I would recommended buying a long established brand and not one that's new and on sale.
That said, I'm also a form believer in carrying a firearm as well. Here is a video of a brown bear attacking a man and getting sprayed by two different people and although the bear got off the man, he looks as if he is ready for more after being sprayed, and this was a bear that was born and raised in captive. Also, if the bear is on you such as if your asleep in your tent, the chance of being able to use spray is small.
Warning, this video contains graphic footage. Bear attack is at around 4:45