Having used both in production:

- https://stripe.com/chat VS email

-You usually don't need to "send the docs in" (being hungarian, they asked for my ID, but email was fine), which reduces time to launch

-After having launched with Paymill, production started throwing strange errors. Root cause analysis revealed, that they don't have USD / EUR acceptance enabled by default; and they asked for extra paperwork to have that. This is not a good problem to have when users are hitting the payment wall.