I know a redshirt that browses the net when bored at home finding funny pic.s and cool conversions.
If you look at CSM cult troops:
Khorne Berzerkers are always on shelf at my local GW, they're made of plastic, $36, 12 models, most popular.
Plauge Marines are usually on shelf, all metal, 7 models, $50, 2nd most popular.
Thousand Sons very rarely in stock, 9 models metal/plastic, $50, 3rd.
Noise marines are never in stcok unless ordered, metal/mostly plastic, 8 models, $45, Last.
Looks like Slaanesh is least popular, judging by the CSM armys I see, yes. I don't get why the tzeentch greater daemon is a bird though, I think it should be a flying octopus.