Option 1: This is more user-friendly for a few reasons. The biggest is probably link sharing and bookmark syncing. If a user on a desktop browser gets shared a link to a m.domain.com, it won't look very good, and non-savvy users will get annoyed. There are also certain users that prefer all pages to be in desktop mode (even on their mobile browser,) so all they need to do is adjust their user agent string on their mobile browser.
Option 2: Some people find this easier, but I can't think of a good reason for it with modern web development. ASP.NET MVC4 makes it really easy to create separate views for the same URL and there's simple functionality to switch between mobile and desktop mode. I would stay away from the subdomain option unless you find a very good reason to use it.