chgoeditor I personally love Essouira, not necessarily for the beaches, which are fine I suppose, but more for the fish market and assorted restaurants right on the water.
I personally thought the ride from Marrakesh to Zagora was pretty gorgeous the whole way. We stopped in Ourzazate just for lunch, but found it nice. Zagora (as you probably know from having been there) isn't too much to write home about, but a trip out to the dunes at Chegaga was a highlight of my trip too.
I'd do Essouira personally, just because it doesn't look like you've been there and it's a nice bit of variety from Marrakesh.
Chris