I am working on a remote sensor project. Optimal battery usage is critical since there is no other source of power. For the finished project, I am thinking about using the 5 volt Arduino Pro http://arduino.cc/en/Main/ArduinoBoardPro or a stand alone Atmega 328 chip.
In other words, the stand alone Atmega 328 chip would be programmed on my Arduino Uno board. Then, I would take it out and wire it to power and a crystal like this https://sites.google.com/site/aaroncrum/aloneatmega328. In terms of saving battery life, I am not sure which is a better approach.