Great question! There are similarities and differences:

Similarities

• Photon is enabled on both
• You have Databricks Runtime on both

Differences

• Databricks Runtime (DBR) version is managed and auto-upgraded in Databricks SQL. Because SQL is a narrower workload than, say, data science, we automatically manage the version of DBR that runs on Databricks SQL Endpoints. This is a good thing - you don't have to worry about upgrading etc.
• DBR behaves slightly differently on SQL Endpoints compared to Clusters. This is again a good thing. Mostly we optimize for the SQL workload and set configs automatically so you don't have to.
• SQL Endpoints are actually behind a scalable gateway proxy. This proxy can, among other things, scale out the clusters as your SQL workload scales up or down. This brings elasticity to your workloads. A bunch of stuff like caching and metadata processing go here, too, to speed things up.

TL;DR if you are doing SQL/BI, please consider using SQL Endpoints, it's generally the best choice for that workload.