You'll save much-much more if you can turn off the screen back light. I believe the phone will last longer with BT headset because the screen is completely off when you use it (though BT itself will consume some additional power).

Regarding the aGPS feature. It's useless in the forest compared to the real GPS. You will need at least 3 towers in range (such an overlapping is rarely used in rural regions) to use it and they are all near the horizon. The GPS satellites are always above you head, just find a bit more open to the sky place. However, Hacksaw is right, aGPS may beat the real GPS in urban jungles - no doubt.

Also I've heard that it is possible to improve the reception with aluminum foil reflector. Though only used the foil once to _reduce_ the signal strength in close proximity to the powerful tower in the park (it was a rare problem with strong signal interference with it's own reflections).