There is a lot of debate in the community over the benefits of the different forms of design for mobile content. The primary options are responsive web design, adaptive design, or separate sites:
Adaptive design, also known as Dynamic serving, use the same URL as the main site but serves the visitor a different website based on the browser. When the web server detects a mobile browser, the end-user gets a mobile version of the web site.
Separate web sites for mobile and desktop browsers use different URL’s to distinguish between the two options. Much like adaptive, the server tries to determine the type of device using the browser identification tag. The difference here is that rather than serving a different page, the server redirects the visitor to another site, usually with a mobile prefix.
Responsive web design is the most popular form of mobile web design. With one single site, you can serve a multitude of screen sizes. The browser will render the page using a set of rules. A smartphone will render the page with smaller fonts, a hidden menu bar, and a small page width. A desktop browser will render the site with a different set of dimension.
Responsive design is also Google’s preference for the mobile web. Google prefers responsive design because it requires less work by the designer. However, there is no single better choice. The only real differences are the cost of implementation.