PayPal is the easiest, so you should start there. As soon as revenues are significant I would go directly with a real payment gateway and merchant account. I've used both authorize.net and Braintree extensively, and both have a secure vault that allows you to build your own recurring payment infrastructure without PCI compliance issues. I have nothing against any of these billing startups—I've never used any of them, and I've set up a lot of payment gateways over the last decade, so take my opinion with a grain of salt—but I have to assume that they charge a significant premium for what isn't that significant of a sunk cost for a web-based startup. In other words, building a recurring payment system is not very hard once you remove PCI compliance from the equation. If you're really short on developer resources then maybe one of these services is a nice stop-gap, but I can't see paying the premium once your revenues are significant.

Again, I'm not really familiar with the value proposition of these services, so anyone with experience (especially a founder of one of these services!) please chime in with a counterpoint.