Originally Posted by mprelaw

I haven't kept up with the GT line in recent years, but, a couple of years ago, the 3, 4, and 5 series all had the same frame. Only difference was the components. Series 4 was Sora, 3 was Tiagra. Tiagra is the better group of the two. The series 1 and 2 had a slightly lighter, slightly more aggressive geometry frame, with internal cable routing. The series 1 used to be only available in the US through Performance.

This may have changed over the last couple of years, so maybe someone else will update it.