The head size is the only difference. By making the head on the D2 wider it moves the COG for a higher ball flight. The D3 has a more narrow head for a lower ball flight. By width of the head I mean from face back.

Matthew is correct here.  The smaller and deeper shape of the 910 D3 will cause a lower launch with less spin than the 910 D2 head.  Everything being equal, the difference is typically a few hundred RPM spin rate drop and about a degree less in launch angle.  But those will vary player to player.

Hope this helps!

I went with the D3 for those same reasons.  Lower spin, and more penetrating ball flight.  Just go get fit.  It's fun and you'll be confident if you $$$ investment and your game.

all the difference translate into lower launch n spin