I would certainly upgrade to Lion. The difference between Lion and Mountain Lion is not exceptional and may not give you too much improvement in performance. A large proportion of improvements in Mavericks are aimed at synchronising your Mac with mobile devices; if you do not use any of these, you may not notice much improvement. However, as time goes by, new apps are often designed only for the latest version of OSX and you may find incompatibilities with an app you have seen and would like to use.