Poll: SMTP or REST for mail integration e.g. Mandrill

Hello, with Mandrill merging with Mailchimp, the threads had many tips on migrating to Sendgrid, Mailgun and others. One important issue for migration is whether one uses SMTP to interface with the mail SaaS provider or REST. REST is often more powerful e.g. has more features, SMTP might be easier to migrate as a standard and poses less of a 'lock-in'. What do you use for mail integration?