On average, the faster glass ought to win in most situations. It will also allow more flexible shutter speed settings and better differential focus/DoF control.
However, as you well know, it is not as simple as that
If you were going to walk around with the camera for an extended period, the 18-70 is lighter. For average shots where the DoF or shutter speed is not crucial, there might not be such a difference.
If you wanted to enlarge/crop/ or crop+enlarge, the pendulum might just start swinging the other way.
Anyway - you have the lens, if you get a D300, you will have the best of both worlds.
