Thanks for writing back and letting us know what you went with!
The one think I would say is that for convenience sake, it might be easiest to either run both lights off a single battery, or get a second charger for the 2nd battery. I store my bike by a power outlet at home, I just hook the charger up to the battery(s) every night. I don't have to mess around with taking them on and off the bike, or trying to remember how long it's been since I last charged them, they're just always ready to go.
On word of caution - don't leave the batteries hooked up to the light when you're not using them (physically disconnect them). They'll drain the battery, even though the light is off. I think it only took about a week to completely drain mine with the light off. (If they're not connected to a light, they'll hold their charge a really, really, really long time though)