Ric,
If your system can afford the power required by the power amplifier going with more power will give you better results long term. We have a nice app note on estimated path loss.
http://www.ti.com/lit/an/swra169a/swra169a.pdf
If you run this code it will estimate the path loss at to be 110dB at 800 meters (good estimate for best case link)
If you run this code it will estimate the path loss at to be 130dB at over 2000 meters.
I would recommend go for more power at 915MHz.
Regards,
/TA