Well there is a very old debate and many SEO will recommend you using sub directory instead of sub domain.

Reasons why you should use Sub directory instead of sub-domain:

• Search engine still counts sub domain as another website so if the blog is in sub directory it will be considered as the part of the website
• Your over all SEO efforts will impact on sub directory which is not the case in terms of sub domain.
• Its way easier to rank for long tail keywords with sub directory instead of sub domain.

If SEO is not a problem for you are you are more interested in branding then i think subdomain goes well as its easier to remember and write directly in the URL bar.