Well, that depends on what you really want the cheaper hard drive for. What is the cost difference? Something else that helps the decision is the hard drive manufacturer.
Here is a link to help you out.
EDIT: It's all about the heat that will be generated also. A 7200rpm drive will give out more heat than a 5400. Granted, I don't know what your current laptop config is like, but if you go with the 7200rpm, find something to help move the heat.