D300 is looking better and better...
Taken from imaging-resource...
Autofocus differences between the D3 and the Nikon D300?
The Nikon D3 and D300 both use the same Multi-CAM 3500 AF engine, but there's been a little confusion over the implementation in the two cameras. For the record, the two systems are identical, but there's one unavoidable difference: The smaller image sensor in the D300 means that the AF area covers a rather larger portion of the overall frame area. This strikes us as an entirely good thing, it'd actually be nice if the D3's AF sensor array were larger, to give the same frame coverage as on the D300.
Other than this one difference resulting from the different frame sizes of the two cameras, the AF systems in both are identical.