Opinion on blog on subdomain vs subdirectory

Hello

Is it good to have the WordPress blog on a subdomain or on a sub directory Please let me know the pros and cons.

Say I hv a blog at http://blog.domain.com and and it has PR 4.
If I manage to redirect all the links from that to http://domain.com/blog, how is it going to affect my PR.

I want the subdirectory blog because hv to create different blog for different section where I can not accomodate category as It has some lmitation.

Please advice me.