When shooting with two different bodies at once, having continuity in the controls/buttons is really helpful. Maybe I missed it, but you're shooting with a D300 already? If that's the case, I'd get a second D300 in a heartbeat. I tried shooting with a D90 as my D300 backup and it's amazingly frustrating when many buttons are different. The D300 is now my D800 backup and fortunately they're almost identical in terms of button layout.