The D2 is more forgiving and will launch the ball higher, while the D3 produces a more penetrating ball flight and is more workable. You need to consider the shaft when getting either of these models. The shaft is the engine that drives the club, so getting the correct head/shaft combo is extremely important. Since you are talking about the 913 series, getting fit for the correct head/shaft combo might not be an option at this time.

What Barry said above and I would get fit from a certified Titleist fitter. That way you can get the correct head/shaft/swing weight.

When I bought my 915, I hit every combination of head and shafts combos so I would definitely hit both and compare your numbers

If I were you, I would prefer to buy the 915 over the 913.
There should be some for sale with the arriving of 917
Between D2 and D3, it is a personal choice after trying both of them.
I prefer the 440 cc of D3 but this is my choice.