Extra battery VS battery grip?

I'm going to be purchasing a Nikon D300s later this week and am unsure what is more worth it: buying an extra battery for the camera itself VS the a battery grip. The grip is more expensive, but would extend battery life.

Which is the smarter choice?

The battery grip is better since it adds some mass to your camera, lens combination. This is especially important when shooting in low light and with a long lens

You will have to buy a second battery in any case to put into your battery grip.

I'm the opposite of Ace. I can't work with a grip well. The pro cameras are bigger and bulkier to begin with. Then add the 70-200 f/2.8 to that and it's a monster. Adding a grip to it was just HUGE for me.
I'm also a female, although I'm not tiny or petite, either. Average I guess.

I'm all for the battery grip.
as said previously it adds weight to your cam increasing stability and correcting weight distribution.
when shooting with a battery grip the first battery gets used and then the second, seamlessly.
should you use a spare battery, you may miss a shot and will dread to see when that battery will die.
plus it looks cool