I use both on free tier and I am happier with PostMark. I had several issues when sending to Microsoft (hotmail, msn, etc.) domains via MailGun because someone

I use both on free tier and I am happier with 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.