Mountain Lion is also a really good version but at least Firefox no longer runs on it.
...okay, so, I will go to bat for there being basically no reason to use Mountain Lion. ?
Mavericks opens up a
lot of compatibility with newer apps, and it dropped compatibility with basically
nothing. Mavericks also added support for memory compression, which makes it
drastically faster than Mountain Lion in memory-constrained situations. When there
is enough memory available, they perform identically.
Mavericks's visual design is identical to Mountain Lion's,
except for some of the included apps, which dropped faux-leather for a more neutral look. Ironically, this means that in terms of visuals, Mavericks resembles Snow Leopard more closely than Mountain Lion does.
I know RobJos mentioned stability issues, which I can't comment on other than to say I've never run into that. I've never actually used the earlier releases of Mavericks though, so perhaps it got cleaned up later on.
By contrast, between Mavericks and Snow Leopard the choice is less obvious. Snow Leopard may or may not be faster on some hardware, and it supports Rosetta, which opens up compatibility with lots of older apps even as it lacks support for newer ones. The visual design is also substantially different, and which you prefer will come down to personal preference—I consider Snow Leopard's visuals to be better in some ways and worse in others. And then there's features like fullscreen and autosave support, which I generally love, but I know a lot of people hate them.
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P.S. Wicknix, we're totally clogging up your thread, I'm sorry! I guess that's how conversations go sometimes...
...and tbqh, I can't miss the chance to discuss Mac OS version difference minutia, which I have very strong feelings about if you can't tell!