They're two different types of bitchy. Shinku is more the aloof kind, like she doesn't really care and is just acting that way out of not caring.
Desu is more the immature kind of bitchy, like the way little kids pick on the people they like. Most people find this cuter, which is probably one reason why people like Desu more than Stinku.
Doesn't really matter though, since everyone loves Junk the most.