Aside from CG adjustability, the primary difference between the Titleist TSi2 and the TSi3 is launch conditions. The TSi2 is a high launch, mid-spin driver, which should be easy to hit high, long, and straight.
The TSi3 offers a combination of mid launch and low spin, which is generally better suited to low handicap players with very fast swing speeds (ie 105+ mph clubhead speed). This makes the TSi3 a bit less forgiving than the TSi2, but it also means that you can get more out of the driver with a well-executed shot.
To summarize this point, it boils down to a matter of trajectory. The TSi2 will give you a higher launch and trajectory, whereas the TSi3 will give you a more penetrating trajectory with low spin.
For a golfer with a moderate swing speed (less than 105 mph), the TSi2 should provide the best carry numbers. However, for players with a high swing speed (105 mph or more), the TSi3 should carry further and roll further than the TSi2.