In my opinion, Mountain Lion is a lot better than Lion in many ways, i.e. for me it is faster, smoother, is better on RAM, has useful features like Notification Center, Notes, Reminders etc.
If your Mac supports Mountain Lion I would upgrade if I were you... but wait and see if it's supported by 10.9 first. If it is, you may want to upgrade to it. If isn't, then you should probably upgrade to Mountain Lion at some point in the near future. (I think you will be pleasantly surprised about the improvements made over lion )
Only you can decide whether the price is "worth" it. However, most opinions are that ML is "Lion done right".
If you think Lion is good, then you'll love ML. There are a lot of bug-fixes and improvements to some of the changes made in Lion.
That machine should be able to run 10.9 without a problem. But Apple may remove Mountain Lion from sale once 10.9 comes out. If you want it at all in the future, be sure to get it when it is still in the App Store.