S.E.O & Domain structures HELP!

I plan to have one main website for my design company (IE. Mydc.com). As far as SEO goes, is it better to have subdomains to promote specials in a specific design catagory or a separate domain (IE. hair.mydc.com or mydchair.com) (IE. clothing.mydc.com or mydcclothing.com)?

Thanks in advance.