Spinning drives are still the "best" for data density and if the IO is sequential (which wouldn't surprise me with AI training workloads), the performance delta may not be that bad vs SSDs. As always, it depends on use case.

I know that a lot of cloud storage has tiered models, where the "expensive, but faster" tiers are SSDs, but then the slower cheaper tiers are HDDs, and the "cold storage" can be HDDs that are turned off all the way to tiers like AWS's S3 "deep archive glacier" tier being tape drives.