GSM is TDMA based...so each "circuit" is assigned a time slice on the tower and a guaranteed swathe of bandwidth to use. The upside is that the bitrate is constant for constant quality (assuming you aren't losing packets). The downside is that there is a very definite "falling off the cliff" effect on the number of clients a tower can service since each one needs a dedicated time slice.

3G is CDMA based...so as more clients connect to the tower, the tower can assign a dynamically shrinking bitrate to all in order to share the available bandwidth and "fail gracefully" as the tower gets more and more clients which leads to more efficient use of bandwidth overall and can handle more clients in general. So, on busy towers, 3G clients will have a downgraded bitrate which will affect the perceived voice quality, but are less likely to get a "service busy" error.