Redshift is based on column store optimized for large, read-only, and complex OLAP / anlytics queries. It supports vertical and horizontal scaling
RDS (Relational Database Service) hosts traditional relational databases (sql server, mysql, postegre), and is designed for transactional workloads (read, write, update, etc.). It supports only vertical scaling (larger node)
Choose Amazon Redshift if you have large volumes of data and need to perform complex analytical queries for business intelligence and data warehousing purposes.
Choose Amazon RDS if you need a relational database for transactional applications or other use cases that require a traditional RDBMS and you're not focused on analytical workloads.