Subdomains VS Subdirectories For ProSites

Hi, I'm going to manually instal MU and ran into a question. I looked it up every way I could, but nope.

For the pro sites plugin which is better, subdomains or subdirectories?

If they are set up as either are there any restrictions as to the number I can create? Yes, I know about asking my host if they have wild card. But are subdirectories just as good?

Thank you, Patrick