I went full frame to the D600, after having owned the D300 and the D50. The main differences between the D300 and D50 is that the D300 is much better at autofocus and low light photography. You should expect good photos at iso1600 on the d300, whereas the D50 has acceptable images at iOS1600. Going up to a d600, I found that I can get good images at iso6400 while the d300 is only acceptable (although sometimes, I was surprised by the d300's low light capability).
I can't discern much of a difference between the d300 and d600 autofocus speeds. As I tend to shoot birds in flight, I would say that you shouldn't see any difference. Just make sure to set the d600 up properly.
A major difference is the weight of the d600 vs d300. The d300 weighs much more...which is great for ruggedness, but bad if you spend a day at the park or amusement park...the d600's low weight is great.
I think the only question that you need to answer is what lenses you have. If you have some fast glass that is full frame, go d600 if you have dx lenses, go d300 (or d300s). I read that the d300s is a little better than the d300 in terms of low light Iso.
However, if you are an enthusiast and want great family pictures,I'd say you should consider getting better glass. I already owned the tamron 28-75 f2.8 and the Nikon 80-200 f2.8 lens. I recently traded my Nikon for the sigma 70-200 f2.8 OS and have been thrilled with the combination.
Last thing to consider, if you plan on getting better lenses, then get the d600, as you can always shoot in DX crop mode with your older lenses and get a 10megapixel picture...which is what I sometimes do with my sigma10-20. This way, you can slowly add lenses to your collection.
Hope this helps.
All I ever really needed to know, I learned from watching Star Trek.
Edited 3 time(s). Last edit at 03/28/2013 04:37PM by wowzer.