Paypal is well established and trusted among the online community. While the Google name is well established as a search engine, it seems to be in it's early stages as a pay system. While it may be less expensive, you must ask yourself if it's worth loosing a client or two to promote Google payments. If you can offer both, you may be able to help your customers become comfortable with it's newness.

Also, if many of your payments are very small you may want to look at Paypals micro-payments, I believe it's still offered.