There are actually four separate products here. There are two "Session" versions which have come bundled with versions of Sonar and their commercial full package versions. Strum Acoustic Session 1.0 came with X3 (and Sonar Platinum). Strum Session 2 comes with the current Sonar Platinum. These are special versions that only work inside Sonar and are lighter versions of the full packages, but actually pretty usable/full featured. (I think you can also buy non-Sonar versions of the Session versions as well direct from AAS).
The full versions are GS-2 and whatever GS-1 was called (can't remember to be honest might have just been Strum). The second generation (GS-2 and Strum Session 2) added a bunch of features at the start (when I upgraded to it/bought it) and was then updated to add in electric models as well. The difference between the Session versions and the full versions is mainly in the number of guitar models. The Sonar Platinum Session 2 version has 2 banks (Acoustic and Electric) and 12 Acoustic and 12 Electric programs. The full GS-2 has 6 banks (Acoustic, Acoustic Experimental, Electric Clean, Electric Crunch, Electric Distortion, Electric DI) and gobs of programs under each bank (50-60 for some categories). The full version also has a bunch of effects that I don't think are in the Session version and a slightly larger loop library.
The version included with Sonar is pretty close to the full version and I think makes a great platform for figuring out if you like it enough to get the upgrade (you may be able to upgrade the Session 1 X3 version to full GS-2 directly at a fairly good price). They have a couple of nice YouTube videos as well that might help you decide if upgrading is worth it. To be honest I'm not a big fan of the simulated string sound. Every time I hear any instrument that is based on simulating strings it kind of hits me as being slightly off. GS-2 is the best string simulation I've heard though. Its real strength is the strum engine and chord system, it is tons of fun to just play. The strumming engine and cord system really do work well, but for me the sounds aren't quite as usable. I do have a long history of buying guitar libraries and instruments and not getting what I'm looking for though, so I'm not sure my opinion is very fair. In a mix and as a songwriting tool though I think GS-2 (and it's lighter Session version) is pretty nice though.
Hope that helps, multiple branched product lines can be kind of confusing.