Bear spray is sometimes effective against aggressive bears of both species and all too often is not. The point about spraying into the wind is an important one and is a major reason why I do not carry spray after testing it.
I have had a LOT of Grizzly and even more Black Bear experience in my working and recreational life and am convinced that caution, alertness and being willing to detour or even leave a particular hike/area is FAR more likely to keep you safe than ANY spray.
In the hands of an experienced and trained user, an appropriate firearm is, IMO, MUCH better than spray to keep you safe. I have a few custom-built Grizzlu defence guns and have been within 10 yards of a Grizzly on three separate occasions, twice armed and once alone and unarmed....I have NEVER had to shoot and have never wanted to, as I am a big wuss where bears are concerned and love them...alhtough they sure could use better hygiene!!!
I seldom carry a gun, DO carry a little airhorn to make noise BEFORE entering an area with restricted vision and am quite confident in my methods as no bear has "et" me, yet, in over 50 years of BC bush activities.
"Bear Encounter Survival Guide", by James Gary Shelton, buy this book and learn what he has to say, it IS the most realistic advice in print that I know of.