The D700 is going to outperform the D300 if you look at image quality. Here are some differences in the specifications in the D300's favor:
The D300 weighs 825g and the D700 weighs 995g without battery. The 170g weight difference will matter if you are backpacking in the wilderness or trekking long distances and want to make every ounce count (then again, you should also consider the D40/D40x/D60 for the weight savings).
The D300 has 100% viewfinder accuracy, and the D700 is 95%.
AF Points Pattern Spread
Both cameras use a similar 51-point Multi-CAM 3500 AF sensor module, which is also found in the D3. The difference is that the D300 uses the DX version for its smaller sensor, and the 51 AF points are spread out across more of the screen. The D700 uses the FX version of the AF sensor module and has its 51 AF Points clustered more towards the middle of the screen.
There’s nothing wrong with the D700’s prism head. It looks like a digital F6. It’s just that I’ve gotten used to and like the stubby prism head of the D300. The lower profile head seems less likely to catch or get hung up in my camera bag.
Anyway, this comparison is just for fun. The reasons to go Full-Frame are strong, and that’s the direction I’m moving towards (just not right now). For me, I’m looking at the lenses I want to use, and the D300 just fits me better. If you want or need a D700, don’t let me dissuade you from getting one, but if you already own a D300, and you do the same stuff as me, then this is some food for thought.
Still to Come: D300 vs. D90
Update 21 Nov 08:
Original article edited due to reader feedback.