I've tried both Stripe and Braintree and in my opinion, Stripe is the clear winner. Their docs are well written and easy to understand. On-boarding is also quick and easy and doesn't require a lengthy approval process like braintree. Lastly, they have the best developer tools and libraries compared to just about anything else out there.

Braintree on the other hand has good international support if that plays a factor into your business.

Stripe has been absolutely flawless in my experience over a couple projects. And it's always instant setup, unlike Braintree IIRC.

I've been using Stripe and they've been excellent, even when encountering issues.

Stay away from paypal.

I've been using Braintree for a few years, and I'm very happy with them.

Which is more important to you?

Speed of implementation, or absolute best price at volume.

If the former, Stripe is almost assuredly the easier implementation. If the latter, Braintree is almost assuredly cheaper (after a fee-dependent threshold.)

stripe, easy to use.