Setting blog in the correct way (SUBdomain, folder structure)

I have seen many blogs. Some websites use SUBdomain like:
http://www.domain.com/en-GB/blog or
http://blog.mydomain.com/

and folder structure like:
http://mydomain.com/2012/05/29/

  1. Should be folder structure related to category like business, marketing…

  2. What is actually the best or correct way as we have also mobile websites?