One is speed, ACS provides up to 5X better performance (latency/throughput) than S3 Express One Zone. We have benchmarks repo here: https://github.com/Accelerated-Cloud-Storage/Benchmarks. Please let me know if you have any questions on this.
Another aspect of this is that ACS buckets are modeled on the traditional S3 bucket structure not the directory buckets of S3 Express One Zone. ACS buckets are not stored in only one availability zone and can be accessed in a performant manner from a region different than the region/az of bucket creation.
One difference is the engineering effort for integration. If an SWE was using Redis they would have to create custom polices on when to move the data between S3 and Redis, how to store it, for how long and when to move newly written data back to S3. This can impose a significant amount of engineering effort for a team. Redis is also mostly single-threaded whereas ACS is designed from the ground up to be multi-threaded.
Another difference is the interface. ACS is accessible through an S3 compatible Python/Go SDK or a high-performance FUSE mount for a POSIX-like file system interface. With Redis, you would still have to build an interface around it for other engineering teams or customers to use.
