I have both balls (not drilled #2 - see my profile) and I think I can give you a reasonable comparison.

The Shift has more length than the Gravity Shift and a slightly more pronounced move in the backend.  That is not to say that the Shift hooks more - it is just a bit more angular for me since it conserves its energy longer.  

The Gravity is a stronger ball overall.  The HD core starts the ball turn earlier than the original shape lock and the R2X pearl cover provides more friction that the R2S pearl particle.  In general my Gravity works best with more oil than the Shift.  For a house shot I'd call the Gravity a medium-heavy oil ball and the Shift a medium oil ball.

I keep the Shift at 2000 and the Gravity at 4000 for the best results matching my game.  If you get both of these balls don't be afraid to tweek the cover to make the balls complement each other.