Redis vs Memcached, what’s the cost impact?

Use Redis. Always. Magento has officially recommended Redis since 2.3.x and the cost difference vs Memcached is negligible at the same memory footprint.

What Redis buys you that Memcached doesn’t:

• Persistent sessions, cart survives a cache restart. Memcached is volatile.
• Two-DB architecture, Magento uses Redis DB0 for cache and DB1 for sessions. Separate eviction policies, separate sizing.
• Page-cache (FPC) layer, you can run a third Redis DB or a separate Redis cluster for FPC, dramatically reducing Varnish dependency.
• Cluster mode, horizontal scaling for stores above 1M orders/year.
• Persistent across deploys, reduces cold-cache penalty after every release.

Pricing reference (ElastiCache):

• cache.t4g.small (1.5GB): ~$25/mo. Good for <$200k GMV stores.
• cache.m6g.large (6.4GB): ~$110/mo. Good for $200k, $2M GMV.
• cache.m6g.xlarge (12.9GB): ~$220/mo. Good for $2M, $10M GMV.
• cache.r6g.large (13.1GB, memory-optimised): ~$140/mo. The sweet spot for most $1M+ Magento stores.

The calculator assumes Redis (it’s the only sane choice). If you’re still on Memcached and on Magento 2.4.x, switch, the migration takes 4-8 hours of dev time and prevents an entire class of session-loss bugs.