SEO Mulitiple websites, with the same backend
 

I have 5 e-store sites all look differnt, but use the same backend to run the store. All are hosted thru differnt companies. we have firgured which hosting company we want to go with, but now have teh problem of, If we should go with subdomains vs. Directories. Also in therory our stores could be in the situation to compeate with themselves, will hosting them with the same company eliminate this possiblity.