Product-wise Mailchimp is nice software. Mailchimp has a free plan which acts as a bait for first time users. Once your contact/subscriber list crosses 2,000 you recognize you are hostage to a very expensive software. Mailchimp is the reason why we started Send With SES. You can read more about it here.
Send With SES does most everything Mailchimp does. I'll make this decision easy for you. You should stick with Mailchimp if you are a happy user of Mailchimp and can afford their high prices. Send With SES does not offer you anything over Mailchimp other than a low price tag.
Mailchimp and many other Email Service Providers (ESPs) charge for the number of contacts you store. Let's say you have as few as 5,000 contacts. Mailchimp charges $50 for storing 5,000 contacts. You pay this fee even if you don't send any emails. Mailchimp offers the myth of unlimited emails. Knock this into your head -- if you send more than 2-3 emails every month to each of your subscribers, very soon you will have no subscribers. They're going to unsubscribe. Nobody likes receiving too many emails -- even if they love your product/service.
By contrast Send With SES does not charge for the number of contacts you have. You pay nothing if you send out less than 3,000 emails every month. Sending 100,000 emails with Send With SES will cost you $30.6 (includes Send With SES charges of $15).