1500mAh battery is pretty great considering the screen size. You'll easily get a day's use and more with that capacity - on both Optimus One (personal vote goes to this, because it comes with stock 2.2) and Galaxy 3. Both are similar in terms of hardware, but Optimus One gets my vote.
LG optimus one (P500), I am using this phone and the battery life is decent enough. The battery last for a day and a half easily; but when you get your first android you tend to play with it a lot and hence you will get a lesser battery life. I am satisfied with the phone; anyway what more to expect from a 11.5k phone.
Never to forget LG has announced Gigerbread release for this phone and this phone is not going to be outdated in near future.