Snow Leopard is a tricky version. If you are trying to install on a Mac that shipped with Snow Leopard, then the retail version sometimes won't work, because the Snow Leopard version that Mac may require is a specific build version, and you can only install Snow Leopard from the DVD that actually came in the box with that Mac.
But, to answer your actual question - I would get the 10.6.3 version, if that is a choice for you.
Just curious - what Mac are you using to install Snow Leopard?