payment gateway integration question

Hi all,
I am going through the process of selecting an e-commerce platform to use for my business (and other clients) - I am currently quite drawn to OpenCart, but have a question I'd like to get cleared up. Obviously the three big Payment Gateways are Paypal, SagePay and Google Checkout. I see OpenCart has support for all three (albeit some discussion about the lack of SagePay Server is something I just noticed).
I would like to ideally use a solution that is seamless and doesn't direct the user away from the site. My current understanding is that Paypal offer something like this, as do SagePay. Do Google Checkout offer this sort of integration or will the user always be redirected to the payment gateway site and then back again.
What are your experiences and recommendations for this? What does OpenCart support and recommend?

cheers,
Baronne

Paypal has "Paypal Pro"
Sagepay has SagePay Direct
Google does not offer an "onsite" payment option

thanks. I guessed that. does opencart integrate with those "seamless" solutions from paypal & sagepay?