Mick . . . I am a long time MusicLab Real Guitar user . . . when I tried Strum Acoustic (the one that came free with X3) I played with it for an hour . . . well, there really was no contest. Real Guitar wins.
Like Paulo says earlier, if you already have Real Guitar 2, then Real Guitar 3 is just a cosmetic interface upgrade, no obvious notable change in the samples.
As far as Sonar 64 bit compatibility, Real Guitar 3 is 64 bit, and works seamlessly in my Sonar X3e install . . . so if your system has been upgraded since you used Real Guitar 2, it's one thing to consider ? (also, in my opinion Music Lab's product support is poor, if that's important to you in a software company)