I think you've already homed in on where the real difference lies between the two...
D300: DX size sensor means your 400mm lens behaves like 600mm and gets you nice and close.
D700: Full frame sensor means you can push the ISO higher and still get acceptable results.
I didn't have this problem when I bought my D300 as the D700 hadn't yet been released and the D3 was way out of my price range. So for me there was no choice.
If I were buying now I think I would still go for the D300 because the reach is more important to me than the high ISO performance. I regularly find myself wishing I could get a bit closer but only occasionally wish I could use a higher ISO.
I don't want to start a fight with Steve (
) but I still don't like using my D300 above ISO 400 as I think the noise levels start to get intrusive. The D700 (and D3) can safely be pushed up to ISO 1600 and even beyond.
Dave P.