I've used both in production and ended up moving everything to Mandrill in the end. Three main reasons:<p>(1) It's easier to take care of my e-mail related customer support issues with Mandrill. They're typically things like investigating missing notification mails or identifying the causes of large daily volume changes. Mandrill's reports and features, like tagging mails with sub-accounts to separate them by app, makes these things very quickly investigated and resolved.<p>(2) Sendgrid customer support did things that were unsettling to me. One of <i>my</i> users was having trouble receiving e-mails from one of my services. He decided to debug that on his own using e-mail headers and talked to Sendgrid directly. As a result, a Sendgrid support agent went into <i>my</i> account and changed a setting without my authorization. The purpose was benign (to get e-mails delivered to this one mailbox), but I was not at all happy with changes being made by someone other than me. What if I had blacklisted that recipient on purpose?<p>(3) Mandrill/MailChimp is older and bigger. E-mail delivery is a relationship business; getting mail through to major ISPs and webmail clients on a daily basis is about reputation and knowing who to talk to when something gets trapped in a filter/blacklist incorrectly. MailChimp's staff has been working with Google/Microsoft/Yahoo/etc for 14 years. They deliver the newsletters for innumerable recognizable brand names. SendGrid's been around since only 2009 and only handles transactional mail; they don't have the same relationship depth or the same clientele.
Use Sendgrid for my previous company (<a href="http://matchist.com" rel="nofollow">http://matchist.com</a>) and using Mandrill for my new one (<a href="http://getdonald.com" rel="nofollow">http://getdonald.com</a>).<p>We used Sendgrid because in additional to transactional email, we needed parsing and events. Sendgrid was just the biggest name at the time so I chose it.<p>All the Sendgrid competitors offer the same functionality for lower prices now, so there's little reason to choose Sendgrid today, in my opinion.
At my previous company we used Mandrill and were extremely happy with it. I like their Web UI a lot.<p>I never used Sendgrid though so I can't compare the two.
I use Mandrill with Mailpoet (wp plugin), and have nothing but good things to say about them.<p>You didn't ask, but we compared them with AWS, and the big problem with AWS was that we had to throttle our own SMTP output. Mandrill will rate limit for you.
We use Mandrill and AWS to send transactional emails. Have never used Sendgrid so I can't comment on its capabilities. But I can say Mandrill has been really good overall and the webhooks make things really easy to tie in.
At my company, we switched from Sendgrid to Mandrill when we ran into some deliverability problems with Sendgrid that we could not identify a cause for, but which magically disappeared once Mandrill was doing the sending.
My previous company migrated from Sendgrid and Mailgun to Mandrill exclusively. Great service, good pricing, great customer support, good reporting, pretty strong all around.
Neither, I really like Mailgun for their receiving email API. Their dev tools make everything really easy to set up and use. For outgoing email, Amazon SES works for me and is cheap.