I have had both, and currently use the D3. The D2 is a little more forgiving, little higher flight, and a little draw bias. The D3 is a more mid flight, hits the ground running, workable, but a little less forgiving. When I say a little, I mean a little, they were basically the same for me, I chose the D3 because I liked the smaller head, and it helped keep my spin down.

Both are excellent head and I can use either one. Try them both out and see which one suits you better.

Forgiveness would probably be the biggest difference with the D2 being friendlier on miss-hits.

Size and shape are the the biggest differences from a visual standpoint.

The D3 has a slightly smaller head with the COG closer to the face which allows you to work the ball a little easier with the D3 vs the D2.

D2 is a 460 CC head while the D3 is 440 CC.
I have the D2 and no problem at all working the ball R to L or L to R.
Whichever one give you the best results is the one to go with.

I have been a long time D3 player, this year I just recently switched to the 917 D2 head. The D2 offers a little more forgiving ball flight on miss hits than the D3, the D3 has lower spin than the D2. To me the 917 D2 with weight placed in toe had similar spin & launch to the 915 D3 with more forgiveness (higher MOI).