I think having a well-aimed taillight is much more important for night-biking than having a spare inner tube and some tools.
I'd get extra mounts and put the D300 on the seatpost. Helmet mounted taillights are a good secondary, supplemental taillight location, but their light can be aimed most any direction depending on how you tilt your head. Tools are seldom used, can be mounted other places than under saddle, and your life doesn't depend on them.
Last edited by seeker333; 10-13-11 at 08:14 PM.