I've been here 6 times! Great place. It takes several days to trek here so if you are comparing it with Erg Chebi - don't. Chebi is reached by tourist bus, in a hotel and there it is, in front of you. Chigaga is accessed via the desert town of M'hamid and then trek from there. 5 - 6 days is about right, total silence, no cars, no roads, no people. Great trekking experience.
Simple choice really, Chigaga for a trekking and true desert experience and Chebi if you just want to see a desert dune from a hotel!