Responsive Web Design
The best configuration for a website designed for most devivces is to use the same URLs and the same HTML but different CSS. This is recommended by Google, the most popular search engine. By using CSS3 media querries, responsive Web design can just alter how these web pages look with the same HTML code.
Of course, Google still support other two different configurations for smartphone-targeting websites: 1)using the same URL but serving different HTMl and CSS according to the different visiting devices. 2)using different URL or even different sub domains ( for example: http://chinapages.com for desktop version and chinapage.mobi for mobile version).
Google told us that responsive web design has some advantages and implied that these sites may be rated as higher quality.
Those advantages include 1)the sites with responsive web design help their visitors to interact, share and link to more easily, thus Google’s algorithms to assign the indexing properties to the contents more easily 2)The Google crawler does not need to craw another set of URLs or act as different user agents to get more or less the same contents.
For more detailed Google recommendation, please check out: https://developers.google.com/webmasters/smartphone-sites/