Which object cache should I use?

Use Redis. It is the only object cache available on WebGee shared, reseller and Premium hosting — Memcached has been retired and the PHP memcached extension is disabled platform-wide.

See Enable Redis object caching to set it up.

What an object cache does

WordPress, Magento and similar applications ask the database the same questions on every page load — which plugins are active, what the site options are, what a given product costs. An object cache keeps those answers in memory so the database is not asked twice.

This is separate from page caching, which stores whole rendered pages. Page caching helps anonymous visitors; object caching helps logged-in users, the admin area, and anything a page cache cannot serve. Most sites benefit from both.

Where it makes the most difference

• A slow WordPress or WooCommerce admin area. The admin is never page-cached, so every load hits the database. See Improve a slow WordPress or WooCommerce dashboard.
• Sites with many plugins. Each one typically adds its own queries on every request.
• Stores with large catalogues, where product and category lookups dominate.

It makes little difference to a small brochure site already served from page cache.

Why Redis rather than Memcached

Redis was kept and Memcached retired for practical reasons:

• Persistence. Redis can survive a restart without every site on the server simultaneously falling back to the database.
• Richer data types, which application caches make real use of.
• Better tooling and maintenance, including per-account memory limits — see Redis memory limits.

Related

• Enable Redis object caching
• Memcached is no longer available
• Use Redis object caching without LSCache