If you have a program like Photoshop, there is little difference, and little point in limiting yourself by using the in-camera black and white setting, unless you find it a convenience.
Original color image.
Original black and white image.
Black and White image derived from color in Photoshop using Image > Mode > Grayscale.
For sophisticated control in Photoshop over black and white images you are best served by shooting in color and using the new channel mixer.
UPDATE 5/14/2009
Better yet is the new Black & White adjustment layer. And for quality black and white conversions, the last thing you should use is Desaturate or the Hue / Saturation slider
thank you, just what I was looking fot.