Firstly what is your product or service? It probably makes a difference who you would choose. For example, if you have subscription payments then Google Checkout isn't in your space (yet). PayPal is an obvious one and I think you can hear "horror stories" about most of them in my opinion - how would your customers perceive you offering payment in PayPal. In theory it could be a negative. Where are you located? Are you wanting to offer in Dollars, Euro, Yen? Give us some more info on what your service/product is and we can help you some more.