I always chuckle when I see the "talk me out of the dark side" threads. If you have progressed to the point where you are ready to work full time as a photographer then you certainly should be able make these determinations on your own.
GrendelZ wrote in post #4164782
I'm not just jumping because this is the flavor of the month, I am going to start building my system around this camera.
But the D300 is the flavor of the month right now. If you can categorically say that this is going to be your body for the next couple of years and build around it on that premise then good for you. If you have the type of personality that is going to tempt you to the next "latest and greatest" then you will be looking back to Canon in a short while. It is said above, Nikon will lead for awhile and then Canon and so on.
I do agree that what you have invested so far is not all that significant and you will recoup some of this through its disposition. It is a matter of lining up your priorities and seeing what from each camp matches those the most. Both companies have good lineups on glass. There will be those that will point out the shortcomings of the Nikon lineup but there are gaps in the Canon lineup as well.
If you are really serious about this question then I would hope that you are posting the same question on a pro Nikon site to provide some balance.