Hey, there are a couple differences between ACS and S3 Express One Zone.

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.