Upload & Sell: Off
I bought a D300 not too long after the camera's release and have been extremely happy with it ever since.
I have wanted to upgrade primarily for dual card slots and video, and to bump the noise ceiling up a bit. Other than that, the D300 was really meeting all of my needs.
The D7100 is an upgrade in every way. The autofocus is another world, the noise has been toned down considerably, the feature set is excellent. It's not a D300 body, which is too bad, but I'm a practical guy - I acknowledge that I miss the D300 body, yet it's hard to argue with the fact that this camera is a better tool for producing photos.
If they ever release a D400, I'd consider an upgrade. I don't think the advice ever changes, though:
- If you need to upgrade, buy the thing that fits your current needs the best, and is available right now. You can always always wait for the next best thing. Unless you have specific information on that next best thing, it's rarely the better choice. If your current needs are similar to the D300, that's the D7100. If your current needs suit full frame better, then it's the D600/800. But FX isn't "better" - it's different, with a different set of challenges.
- If you don't need to upgrade, don't.