Last year i went through this very question.
I would suggest you take a memory card with you and go to a camera store that has both and make test shots with both . Use the same focal length, etc. then go home and view the results on your monitor, NOT THE BACK OF THE CAMERA.
For myself, if i was going to be using basically jpeg files i would have gone for the d300, as i liked the results better than the d700; however, after a lot of teeth gashing i went with the 700 and love the results and 95% of the time I am using RAW files so my personal preference for jpeg files does not come into play.
Also , be sure your using a lens you own now, not a high cost one that is in the case.
I wish I had a store around here that had high end cameras like these... The only thing we have in Savannah is a Wolf Camera, which doesn't carry anything higher than a D90.
As far as subjects go, I do occasionally shoot when light conditions aren't the best, but for the most part, I do shoots in well lit areas.... As of lately, I've been getting a few side jobs doing family portraits and things like that... How would the 300s compare to the 700 in that respect.
I have a nice lens that I have been using with my D5000, a Nikon 24-70mm F/2.8. So with that being said, would the 300s suit me well? Or should I just hold out for the 700?