Sub domain or subfolder which is more SEO friendly?

I am little bit confuse on sub domain and sub folder, which is the best choice.
1. My website all ready have page rank 2 .Now I want create some more section in details. Should I go for sub domain or sub folder? Which one is better?
2. My another question is did my present page rank will follow in all my sub domain or I have to promote individually all sub domains and what is case in case of sub folder ?
3. What you thing which is easy to remember sub domain or sub folder from the point of view of visitor?
Thanks in advance.

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