I favor the spare Drive for the following reasons:
1. It can be put into service immediately -- no repair, tools or delay necessary.
2. The kit does not cover all possibilities; the spare does.
3. Repairs on the water are difficult under the best of conditions; impossible in foul weather. There is a risk of losing parts.
Keep in mind, frequent inspection and adjustment of your Drive prevents most failures. With almost all failures, the Drive is still serviceable and will continue to operate at some level. This includes broken/lost mast, detached fin (not included in your kit), broken chain/cable, striped drum (not included in the kit) and broken pedal/crank arm (not included in the kit). You can simulate these failures by disconnecting various parts and trying it for yourself.
Having said that, I feel the current Drive is very reliable, but have experienced all the failures mentioned at various times in the past (pre V-2).