In the end the 40D won thanks to the 100-400L and Canon's recent
aggressive pricing. So the 40D was almost half the price of the D300,
leaving me plenty of cash free to buy spare batteries.

Fortunately the 40D has some very cheap and well regarded spares
available. So I bought three, just to make sure I'd have enough juice for
a long day out.

You should of done some research before you bought spare Li-ion
batteries. They degrade from time of manufacturing, even without
being used.

http://www.batteryuniversity.com/parttwo-34.htm

From the link: "Avoid purchasing spare lithium-ion batteries for later
use. Observe manufacturing dates. Do not buy old stock,
even if sold at clearance prices. "