They're not the same use case, I admit. MailChimp is more of a broadcast / marketing email platform, with elements like campaigns to consider, whereas Mandrill is a transactional email service, for personalized delivery of email messages like notifications from an application. I'm not surprised that they have somewhat different feature sets as a result. 

I was a little surprised to discover that Mandrill and Mailchimp have a totally different account system, and that while you can link your MailChimp account to your Mandrill account, it's not like having a single account with multiple products/services.

The more you look, the deeper the differences get, and the more surprised I was.

For instance, look at templates. MailChimp allows you to define email templates that you might send to a mailing list. Think of something like a sale email, where you might configure a template and then merge in a few specifics as you're sending to each person. MailChimp has a sophisticated WYSIWYG editor that you can use to define these email templates.

Mandrill also has templates. You might want to configure a template for the welcome email for your application in Mandrill rather than having the application encode all the details of what's in the email. This means that later, you could come back in and modify the template without changing the application.

However, Mandrill doesn't have a WYSIWYG editor. If you wanted to have someone on your growth hacking team change the email, they'll have to know HTML to do it. That may not be the end of the world, depending on your company structure but it was a surprising inconsistency for me, and a bit of a missed opportunity.

There are integrations between the two. For instance, you can actually create an HTML template in MailChimp using their editor and then send the template over to Mandrill. But when you do so, you don't seem have the option of choosing which template to replace, or to make use of Mandrill's template versioning. And if you make use of Merge Codes (think personalized fields) in MailChimp's designer, some, but not all of those codes are supported in Mandrill, but neither MailChimp nor Mandrill seems to mind if you make use of them and then send the template from MailChimp to Mandrill. Those codes won't do their job anymore, but neither system tells you that, from what I can see.

I just think MailChimp has missed an opportunity, both to integrate these two systems more tightly, encouraging MailChimp customers to consider Mandrill and vice versa, and also, frankly, to improve the experience of both systems by leveraging the work done by the other. Why doesn't MailChimp offer their WYSIWYG designer in the Mandrill interface? Can MailChimp templates by sent to Mandrill to become alternate versions of existing templates rather than replacing them? If not, why not?