The verdict
If you already have users and your emails to them are scattered across two systems, I'd pick Loops. It brings onboarding, lifecycle and transactional email together and gives you one set of rules.
If traffic is fine but sign-ups are weak or sales call the leads thin, pick Outgrow. An ROI calculator or readiness assessment gives visitors a useful answer and gives you qualification data.
For most SaaS teams, email to existing users is the better first fix. Add Outgrow when you know which page needs a stronger lead magnet. Check the integration list first, since Outgrow lists API access as a custom feature.