url structure for mobile/desktop site

What are the pros and cons of these url formats for a website that does mobile and desktop content.开发者_开发知识库..

• mobile.example.com
• example.com/mobile
• no explicit url, but send back dynamic content based on browser, or querystring variable?

thanks

W3 recommends "When accessing site entry points users should not have to enter a filename as part of the URI. If possible, configure Web sites so that they can be accessed without having to specify a sub-domain as part of the URI."

So m.example.com or example.com/m would be best solution