Why should I choose the D2 instead of the D1?

It depends on what you shoot and also on your future ambitions. If you shoot fast-moving subjects and need to freeze that movement you should have the D2. It has extremely fast flash duration for freezing action in low light situations and HSS to freeze in bright conditions. If you shoot in mixed light situations or if you go to places where you can’t control the ambient light you should have the D2 since it offers HSS with full control of ambient light If you need to get that perfect moment that is only available in a fraction of a second, you should get the D2 since you can shoot up to 20 flashes per second. If you need very little power (e.g. shooting close to subject, large aperture, high ISO, etc.) the D2 is a better choice since it has a very wide 10 f-stop energy range. With the D2 500 you can go down as low as 1ws! The D2 has a new and improved cooling fan which is more silent than the D1’s. If you shoot fast-moving subjects and need to freeze that movement you should have the D2. It has extremely fast flash duration for freezing action in low light situations and HSS to freeze in bright conditions. If you mainly shoot portraits or still images in a controlled studio environment and don't have any other ambitions, then the D1 could be the choice for you.