120 grams eh? That's noticeable, but not a dealbreaker. The entire thing shouldn't be too much heavier than the DS: 275 grams. PSP is 280 grams. So I think total aim should be about 300. Of course I don't really know what even 600 grams would feel like, the DS is pretty damn light.
So the question really relies on how heavy the entire thing would be, and how much would a light battery weigh? If we're talking about a difference of 50 grams, I'd definitely rather have the heavy one. If we're talking about 100 grams, I'm not sure. I guess what I'm asking is: What would be the ratio of weight to life?
16 hours? Damn that's long. That'd be amazing
EDIT: Just did a bit more research. Apparently a AA battery is 31 grams, so the battery you mentioned would be approx. the weight of 4 AAs. That's really light... I wouldn't foresee any objection. Heh, for 16 hours...wow! That's a pretty huge number!
Something that's important to me would be
charging. I know that lots of rechargeable batteries die if you
use them plugged in. Apparently the current would go through the battery to get to the device. Will the Pandora have this problem? If so, maybe you should consider having the battery easily removed (no screwdriver required) so that you can take it out if you want to use it plugged in.