If you're working with .NET 3.5, by all means use SubSonic 3.0. If you think you're going to need to work with 2.0 ever - use 2.x.
Considerations== 2.0 (and subsequent point releases) are by far the most stable codebase SubSonic has. 2.x has been in the wild for over 2 years and has reached a nice stability point. 3.0, on the other hand, is very, very new. Having been released in June of 2009, it was under development for almost a year and has undergone a lot of testing on projects and Real Life. That said, there may still be some lurking bugs in the core since a lot of it was streamlined and rewritten. ==Support
Both of the libraries are supported by the community in the form of patches and contributions. SubSonic's main contributor and Chief Overlord, , works on bugs when he can, and loves to have people contribute patches to fix the bugs in the butter.
Pros and Cons of 3.0
3.0 uses Linq, a lot, as well as many of the C# 3.5 language features - such as Extension Methods and Expressions. If you're not comfortable with any of these, 2.x might work better for you. If you are comfortable with them, then you can save a lot of code using the shortcuts the new features provide. A long query can be cut to 1 line using an Expression set, for example. Whichever you choose, our goal is to get you home earlier - let us know how we're doing.