Morocco has two great sand seas. They are completely different experiences.
Erg Merzouga — Closer to Marrakech, heavily touristed, good infrastructure. Fine for a one-night introduction to the Sahara.
Erg Chigaga — Morocco's largest dunes. Remote, raw, almost entirely free of mass tourism. Requires a 4x4 approach or a multi-day trek. This is our home.
Verdict: If you want authentic — Erg Chigaga via M'Hamid El Ghizlane. If you want convenient — Merzouga. You can only choose one.