At this moment, Mountain Lion is a more mature, refined OS, having a .4 update, and .5 coming soon. Whereas Mavericks is still in beta. Now, I realise, when you read these forums, people are saying that Mavericks, even in beta, is very stable. But to put that in perspective, on these same forums you'll find similar comments on the stability of Mountain Lion's betas - whereas, when Mountain Lion shipped, there were bugs.
Since I use my Macs for business, I like them to be as trouble-free as possible. I've used the recent OSX Macs - Tiger, Leopard, Snow Leopard, Lion and Mountain Lion. I say, you cannot go wrong waiting till each OS has reached at least .4. I used to say till it reached .6 (since, in some earlier iterations, some major bugs weren't fixed until .6), but with Apple's fast tracking of new OS's, it's getting to the point where .4 is rather mature, whereas years ago, .4 was mid-life rather than old age.
So my recommendation would be to install Mountain Lion now at 10.8.4 - and wait till Mavericks .4 update some time middle of 2014. You cannot go wrong.
And always do a clean install, rather than an upgrade.
And if you do upgrade, download the Combo file from Apple's website, i.e. do not use the App Store software updater.