Few years ago I would say that it heavily depends on the level of autonomy of different sections of a site, what is a measure of being separate. If user most often enter the site through typing main url, it can be better to prefer pathnames instead of subdomains.
But today modern browsers autocomplete url from any kind of substring, so this issue is mitigated.
The rule of thumb is nevertheless relying on degree of separation.
Say http://site.com/mysettings is almost always better then http://mysettings.site.com
On the other hand, http://site.com/mymail and http://site.com/myrssreader are worse compare to mymail.site.com and myrssreader.site.com
Just because mail app and rss app are sO different that they would need one additional level of pages and http://site.com/mymail/settings is hardly better than http://mymail.site.com/settings.
As for redirecting from subdomains to pathnames or vice versa, I'd rather not.