MailWizz. It was rebuilt from the ground up for modern multi-SMTP delivery.

unless you have a specific reason not to. It is cheaper, faster, and more modern. The only reason to pick Interspire today is if:

MailWizz (by a mile for raw speed).

When choosing between and Interspire Email Marketer (IEM) , you are looking at the two most dominant players in the self-hosted email marketing space. Both offer the "Holy Grail" of email marketing: a one-time license fee and total control over your own data and infrastructure.