XBee,..Which Addressing mode to use

I have two Series1 XBee modules .. How to decide which addressing mode 16-bit or 64 bit to use? what parameters are to be concerned ?

How to decide which addressing mode 16-bit or 64 bit to use?

Your choices are 16 bit or 32 bit. If you are addressing more than 65,535 XBees, you need to use 32 bit mode. If you are addressing one other XBee, then you can use 16 bit mode.