My estimation is that brown bears and grizzly bears are much more dangerous than black bears. They are larger, more unpredictable, and more lethal. Black bears can be problematic because they are very intelligent and very curious, but not so lethal. Black bears seem better at solving problems, opening things, and getting into mischief.
I would never consider wasting a perfectly good can of bear spray on a black bear unless it already had my leg in its jaws. For a grizz, though, I might value having a second can of it handy.