Those are the words I don't hear much, but not that I've never heard of them either. In my assumption the two words are synonymous and may be exchangeable? I think stud either mean someone, usually a guy who is either really cool or sexually appealing? Which I thought stud muffin means the same.
Most Helpful Girl
Stud is someone muscular and very manly
Stud muffin is everyone else that's hot1