At this point... id keep running SL if you are happy with it.
Lion vs ML? A couple of new features in ML. ML was way faster than Lion pre 10.7.4, but now i'm not so sure there's a huge difference.
If your machine will run Lion, it will probably run ML. it is no heavier on RAM than Lion is, and I actually think its a little less hungry. I've got a copy running in a VM on my MBP with fairly stingy memory settings and it runs just fine.