My app is very small, just bootstrapping without any funding so I'm trying to be very careful how I spend my money. At first I see that Mailgun's minimum was $19/month while Sendgrid's silver plan is $10/month. Easy, Sendgrid wins.
Oops, I want to support inbound email though. Sendgrid will give you that on the trial plan, but not on the $10 silver plan. Suddenly I'm up to $80/month to use Sendgrid. Mailgun wins here.
But before I even noticed that I had already signed up for Sendgrid (believing I'd be paying $10/month), but had to wait like 5 hours and then I received some email from them basically demanding that I tell them what my app is, why I need email, what is the purpose of me sending email. Maybe it's for market research, but it didn't come across that way. It came across as "we assume you're a dirty spammer". Of course if I was a spammer I'd just give them so BS answer, I wouldn't admit it. I can't say call a winner, only a lower: Sendgrid. Mandrill and SES both treat you like you expect; you sign up, they cap you at ~250 emails/day for a bit and then you're fine. Sendgrid fail.
Anyway, then my friend points me over to Mandrill and I sign up. The pricing model fit us well.. up to 12k emails free, then $0.20/thousand after that. In the state my app is right now I probably won't go over 12k for another few months, and once I do go over that limit I'm not going to find myself jumping suddenly to something like $80/month.
If I had been setting this all up just a week later (that is, now) I'd probably be on Mailgun due to this new pricing model.