I'm guessing that the smaller head, the D3, increases the distance on a better hit, but the D2 has a little more mass so it is more forgiving.  I would like to know how to choose between the D2 and the D3. I have the 910D2 but I've been wondering about the D3.

The D2 is more forgiving. If you want to work the ball more, the D3 will be much easier to do so with. The difference in distance will not be much, the main difference in the size is going to affect workability the most.

The size will have nothing to do with getting more speed/distance.

The D3 will be a smaller head obviously by 15cc, with a more pear shape. It'll give a mid/high launch with med/low spin.. Very middle of the road head for most people. Pretty darn forgiving, IMO, and I find after being fit, that the head is longer than anything I've used, but not by much. Feels and looks better.

The D2 is  a slightly larger head, more round. Will be slightly more forgiving, and COULD be longer for most due to the higher launch, and higher spin. The higher spin is what keeps most better players away, as they fight to keep the spin low with the harder swing speeds. Most people get more distance the higher the ball, as it gives them better carry numbers before the ball falls out of the sky.

Go demo both clubs and see which gives you better numbers. I found the D2 to be a larger footprint, but pretty easy to look at. It gave me around 400-500 more RPM's of spin than the D3, and was a bit higher launch than I liked. I went for the D3 as I was able to flatten the lie, and open it a bit, giving me the ability to not go left, keep my launch lower and spin much lower as well.

Guys Check out Gators post on  D3 Launch and spin thread.  Could answer your questions

Deno

HI James, The D2 is a little more forgiving then the D3 But the D3 is a little more workable then the D2. If you like to shape your shots then I'd think the D2 would be better. I went to a Titlest Advanced fitter and hit both equally well I'm right handed and could cut both heads equally well but the D3 was just a little easier for me to draw. I went with the D3 but that was mainly base on the look of the head at address

I hit the D3 farther.  I was originally playing the 907D1 (460 CC) and figured I would lose distance.  The cary distance was just a touch longer with the 910 D3, but the total distance was better.  You can't really go wrong with either one.