For me, the deciding factor to go with a D700 was the low light high ISO performance. Up to ISO 6400 I find the D700 noise free with well exposed images vs ISO 1600 for the D300.
If I need the advantage of crop factor, the D300 still see's quite a bit of use.
There is a distinct difference in the appearance of images between the two bodies, with DOF being the noticable difference. Along with the perceived increase in magnification of a lens with DX, there is a corrisponding increase in depth of field also.
Whether or not its worth owning both bodies or upgrading to one of the newest generations, is a decision only you can make.

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