If you're happy with Snow Leopard, I would recommend just staying with it. It was the last version of OS X that I could run on my Early 2006 MacBook Pro, and as my 'new' machine came with Lion, I can't attest to whether Mountain Lion would faster or slower or what on your machine.
If you do decide to upgrade, I would make a clone of your current drive using 10.6.8 so that if you DON'T like Mountain Lion you could always revert back to Snow Leopard.
The change from SL to ML is a major OS upgrade and has many pre-requisites and consequences, so it is advisable to do some homework. There is a lot of help available both from Apple and elsewhere, so check out your computer is up to it and also the software you may need to upgrade too, and how much that may cost.
Here is one of many articles that may help.
Edit: Good advice above, on making a clone of your present system before jumping in!
On the computer that has enough memory I don't think lion is any faster than Snow Leopard and if your computer doesn't have enough memory you'll find that many programs run significantly slower in lion. I think Mountain lion is a little bit faster than lion was but it also requires a little bit more memory. If your computer has 4 GB or can be upgraded to 4 GB lion will run okay but without 4 GB don't consider upgrading. Also if you choose to upgrade, before you do so make a complete fresh back up and make sure that all of your software is up to date.