Start with PayPal, add a merchant account once your volume makes it worthwhile. Billing code is really not hard to write, at least in the beginning when you're not dealing with a lot of special cases yet. With PayPal subscriptions, you basically just need two things -- some code to generate a link to start the subscription (which you can then put in a webpage or redirect to after someone fills out a form), and a script to receive the POST-backs from IPN when subscriptions are created or canceled. And that's all part of their standard service, not the Pro/PayFlow plans with monthly fees... why commit to monthly fees for Chargify or Recurly or anyone before you even have revenue?
I tried the subscription-as-a-service stuff before, with both Spreedly and Recurly, and ended up with just as much work and just as much code, while tying myself to startups that are much less stable than PP. That came back to bite me when Recurly screwed their beta users on plans/pricing and I had to rip out all their code and start over.
