I play the D3 10.5, I prefer the look of the more compact head shape. I also like the lower launch & spin, this allows me to go to the higher lofted driver while helping a little with the spin numbers. I was told that the forgivness between the 917 D2 & D3 was minimal.

I have a D3 although only played one round with it and that was prior to having the grip adjusted to the right size so wasn't comfortable with it. Been to the range since then and feels much nicer.
Went with the D3 as the headsize is similar shape to what I've had for the past 12 years. Have tried the larger headed clubs in the past but don't like the feel of them, they felt as if they dragged through the air (Probably don't at all but that's how it seemed so put me off).
As everyone else has said though, get yourself fit for it and they will prove to you with all the data at their disposal which is the best club for you.
Good luck with it all and enjoy your new 'Big Dog' when you get it!!