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You need to know that the DOF of 50/1.4 wide open used on D300 is not at all shallower than the DOF of 85/1.8 wide open used on D700.
Also you need to look for the quality of bokeh... from what I saw posted by others 85/1.8G provide a more pleasant bokeh than the 50/1.4G, a lens I owned in the past w/o to impress me. If you want to go for a 50 you better choose Sigma 50/1.4 for a better character and bokeh.
So, in your place I'll pick 85/1.8G or, if manual focus is not a problem Samyang 85/1.4 is a steal for the money.