They are two totally different services. Mailchimp lets you compose campaign emails. Whereas Sendgrid send transactional emails. Mandrill is Mailchimp's service for transactional emails.
I am aware of the difference of MailChimp the service, and Sendgrid, but MailChimp the Company owns Mandrill which is the equivalent of Sendgrid.
My question was which company is most likely to sponsor a tiny startup.