Minutemailer is a strong tool with an emphasis on visual emails and "what you see is what you get"-based email building. They've got a strong record when it comes to their commitment to privacy and GDPR compliance.
Overall, they're not a terrible choice (especially if you're based in the EU, which is a core part of how they position themselves.) Buttondown offers three main things that Minutemailer does not:
| Minutemailer | Buttondown | |
Minutemailer —  | Buttondown  | |
Send from custom domain  | Minutemailer —  | Buttondown  | 
Minutemailer —  | Buttondown  | |
Minutemailer —  | Buttondown  | |
CSS  | Minutemailer —  | Buttondown  | 
Minutemailer —  | Buttondown  | |
Tags  | Minutemailer —  | Buttondown  | 
Metadata  | Minutemailer —  | Buttondown  | 
Minutemailer —  | Buttondown  | |
Minutemailer —  | Buttondown  | |
Minutemailer —  | Buttondown  | |
Minutemailer —  | Buttondown  | |
Minutemailer —  | Buttondown  | |
Minutemailer —  | Buttondown  | |
Minutemailer —  | Buttondown  | |
Host on custom domain  | Minutemailer —  | Buttondown  | 
Minutemailer —  | Buttondown  | |
Minutemailer —  | Buttondown  | |
Minutemailer —  | Buttondown  | |
Minutemailer —  | Buttondown  | |
Minutemailer —  | Buttondown  | |
Minutemailer —  | Buttondown  | |
Dedicated IP  | Minutemailer —  | Buttondown  |