I shoot a D700. It is my understanding that the image quality is virtually identical, in user reports and lab tests. The AF is said to be "better" on the D3 but those things are hard to quantify (?). Supposedly it's AF engine is a bit more speedy even though it uses the same basic CAM3500FX AF sensors.
Assuming you are comparing apples to apples - new to new or used to used, and you like the D3 integrated grip, verses the flexibility of a smaller, lighter camera plus grip when needed, there are a number of advantages to the D3 that I think can easily be well worth $200-300:
1. An MB-D10 grip alone is $250 or so and if you always shoot with a grip, the D3 integrated grip will give slightly better performance on a tripod (eliminating a connection joint) and it may feel better. The D3 is slightly smaller than a D700. A D700 with grip can be problematic in camera bags designed to just barely wedge it in, and my experience is that there are a lot of them. Especially the Think Tank stuff I have.
2. An MB-D10 works best with EN-EL4a batteries. That adds another $250 to the cost. You can use AA's with the MB-D10 grip and still get 8FPS but AA's are more work and don;t proivide remaining life as effectively. Conversely with the D3 you lose the internal EN-EL3 "backup battery".
3. D3 will give you 9fps verses 8fps with the grip and some say it is more responsive than the D700.
4. Having owned a D2h for a long time, the extra real estate on the "grip" is nice, espeally the extra LCD screen and more instant access controls.
Conversely, the D3 forces you to buy an SU-800 or use an expensive Speedlight in the hot shoe, just to fire off camera flashes in commander mode. The D3, though, is arguably more rugged without the pop-up mechanism.
If you are a grip kind of guy, and you did not allow for that in the price difference, I would think that would seal the deal. Similarly if you want the very best in difficult AF action shots, the D3 might be worth it for that alone.
My D700 is my poor man's D3, but back then the diff was $2500, not $300 and I already had a pair of EN-EL4's. At $300 I would have been all over it, even though I do like the D700's "convertible" option to a small footprint camera. Even if the D3 was $300 more than a D700 with grip and EN-EL4a and charger, I would still go for it.