i would decide by:
1) what is your installed base?
2) which one are you currently most comfortable with?
3) what are the people who will have to maintain the equipment more comfortable with.
4) aside from this project, getting more experience in which product line is going to help you yourself down the road? (normally ab in us and normally siemens in europer)
i personally don't like the s7 development software at all. I could go on and on for hours about why, but i will spare you. Generally speaking they have the same capabilities. If you are starting from ground zero and have almost zero plc experience you will almost certainly be able to be more productive with rslogix5000 in a shorter amount of time than you will s7. S7 requires a lot more up front effort and knowledge.