Hello t_hyatt.
First I will ask this forums moderator to move your post to the Help forums so it doesn't get buried here with no one seeing it.
There is more than one pro and/or semi/pro here using the D300. I don't know why the pro advised the amateur not to buy the d300 unless he thought it would be too much camera for him or perhaps thinks that all pros should have full frame cameras or something.
If you already have Nikon lenses then I think you definitly will want to stay with Nikon.
They are the priciest investment we make in our outfits and the ones that last the longest too.