Most stable/fastest OS X version for an old MacBook Pro
My brother owns a Mid-2008 MacBook Pro and installed a fresh Mavericks. In his opinion, it is worth doing this since Mavericks' performances are better than the Lion and ML releases.

However, he did replace his HDD with a SSD drive to make its MBP fast enough to run Mavericks.

So I cannot encourage you to install Mavericks on your older MBP without changing some components (and at least remove some stuffs from your hard-drive to get more space).

About the Mavericks bugs: the two updates made the things better, even if there are still some annoying issues. But that should not be your choice criterion.

My advice


Make a Time Machine backup of your MBP and give a try to Mavericks (with a fresh install).
If it is too slow on your MBP , you will have to re-install a fresh OS X 10.6, but this will probably be a good thing: having a fresh install sometimes solves some annoying issues.
if it is OK for you then let us know your experience