Is Merzouga better than Zagora for big dunes?

If big dunes are specifically what you're after, then yes — Merzouga is decisively better than Zagora. Merzouga borders Erg Chebbi, a proper dune sea where the sand ridges climb to around 150 metres. These are the towering, sculpted, golden dunes that define the Sahara in most people's imagination, and they photograph and feel exactly as you'd hope.

Zagora's desert is a different character altogether. Its dunes are small, lower, and more spread out, set in a stonier, flatter landscape closer to hamada (rocky desert) than a dune field. It's still 'the desert' and still has its charm, but anyone expecting the dramatic dune scenery of the photos will likely feel let down at Zagora.

The only thing Zagora has going for it on this question is access: it's roughly 7 hours from Marrakech versus about 9 for Merzouga. So Zagora exists primarily as a time-saver, not a scenery upgrade. If you choose it, do so knowing you're trading dune size for a couple of hours less driving.

Bottom line: for the big, classic Sahara dunes, Merzouga wins without much debate. Make the extra effort to get there if those dunes are the reason you want to see the desert at all — it's the difference between the Sahara you imagined and a more modest substitute.