Mike, you may be disappointed with the control differences on the D600 from the D300.
If the cash isn't an issue, consider a D800. The D800 is much more similar to the D300 in terms of size, control layout, and control presence (eg an AF-ON button). If you'll be using both cameras at the same time with one lens on each, these similarities can really make a difference. If you'll only shoot one camera at a time, then you might not mind as much but I promise you'll notice and have to really think about it when switching bodies. The D300 and D800 are pretty seamless to swap between I've found.