I can't which one is the best. But we use MailGun and we are not happy with it. We will probably switch to Postmark.
I had several issues when sending to Microsoft (hotmail, msn, etc.) domains via MailGun because someone exceeded granted quota for outbound IP address and since then I balance outgoing emails using both.
As mentioned by others, MailGun support doesn't care much about queries from free tier users so I would go for PostMark if I had to pay (more intensive use) or recommend to someone.
Is there any specific issue you ran into?
I'm using their REST api to send emails, and I've yet to see an email take more than a couple of seconds to get to destination (gmail) while testing.
How long is too long?