A good starting point for learning is the
Wiki.
There you will also find links to the various versions.
As for which version to use - Start with the official 2.6 and get familiar with it.
If things move too slowly, you can try out SEt's 2.6 MT which allows you to use multiple threads and can speed up thing considerably, assuming you have a multi-core CPU. However, it also potentially introduces problems like crashes due to memory limitations (although these are often caused by incorrect use).