MemoryDB: 30% cheaper, 10TB free writes per month and just $0.04/GB (compared to $0.20/GB with the Redis version) after.

And simple, no-downtime upgrades for ElastiCache and MemoryDB.

From my research, MemoryDB is the only actually ACID-compliant Redis-compatible DB engine (using WAL to multi-AZ storage, similar to Aurora) with the same read performance as standard Redis. Standard Redis replication fails Isolation and Durability in failure edge-cases.

AWS really wants you to move away from Redis.