I've just updated my (late 2009) iMac to Mountain Lion. It's now running like a dream. So far, so good.

Mountain Lion <<seems>> to have fixed whatever the hell Lion did to it. It started running like a dog after putting that on.

That said, the MacBook seemed OK with Lion, but that's not so much used.

If my initial experience with Mountain Lion (which also looks really nice on the MacBook, btw) is any indication, then I'd say well
done for skipping Lion, but you should probably go with this one. Many of the half-baked features of Lion seem much better cooked now.