Strapi is a headless CMS for managing content and creating easily customizable APIs to power applications using that content. Loopback is a framework for building APIs and fully bootstrapped microservices. While Strapi is batteries-included and does solve a lot out-of-the-box (Auth, Permissions, Code Generation, etc.), most of these use cases revolve around content provided on Strapi admin and providing APIs for consuming that content easily. Since your use-case is of a CRM, I would assume that you'd have to write a whole bunch of business logic rather than just collecting content/data via Admin and rendering the same.

I'd suggest thinking along the lines of the below use cases that you may have now or in the future and then finding the right tool for the job.

1. Users, Roles, Permission (Authorisation and Authentication)

2. REST and WebSocket scenarios

3. Multi-tenancy and data sources (Single / Multiple DBs) (Revisit 1st point)

4. Caching

5. Async processing (Kafka consumers)

6. Periodic Activities (Crons)

7. Scaling strategy in case of heavy load (inter/intra-requests)

8. Internationalisation

9. NFRs

Strapi / Contentful would be a no-brainer choice for a frontend-heavy application but use cases like CRM are essentially backend-heavy and are seen as a SAAS product in itself than just a website.

I'd recommend you check out the likes of NestJS / SailsJS once.

Both of them would work for you. Strapi is easy to get up and running in no time. Loopback would require a bit of fiddling but offers more control. So it depends, you can go a long with with Strapi and it would work out of the box for most of your apps functionality and it offers a good UI. But for some custom features you'll have to do an extra mile to make things work. For loopback it takes a while to get comfortable with but then it's a breeze.

I'd personally use Loopback but again it all depends upon your team and what you guys are comfortable with.