I run Lion on an iMac 27" i5. I like it very much, especially the finger gestures with the magic mouse or trackpad, using Safari or Chrome. Also upgraded a macbook with i5 and an iMac core2duo from snow leopard to lion through the appstore. Made an extra backup, but it went without a hitch and it wasn't any slower. There was one problem with erratic WiFi on the macbook, but an extra automatic update fixed that..
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.
It has always had 8 GB. It was great with Snow Leopard, but Lion just seemed to be so sluggish, not to mention so many early problems with WiFi, especially, crashing the machine. Although, my MacBook (mid 2011) never exhibited either of these problems.
But I tell no lies when I say that after installing Mountain Lion, my iMac really feels like a new machine again. It's snappy and responsive again.
After my experience with Lion, I vowed NEVER to upgrade an OS on day one of it's release EVER again. But I couldn't help myself And on this occasion I'm glad for that!