Google delivered the opening address at Moboom's 2013 User Conference in London. In this clip, Google's Marcel Gordon discusses why responsive web design is their preferred approach for mobile.
Google famously doesn't say much about search, or give much insight into how their algorithms work. So a lot of heads were turned this year when a help center article went as far as to say they prefer responsive websites.
Gordon elaborates more on their recommendations, describing the benefits of responsive for publishers, advertisers, and mobile SEO.
For publishers, having one server, one set of content, and one front end is a "big step forward in terms of maintainability." He gives the analogy of running a bundle of cables through a house versus separate cables—it's much more efficient to maintain one system.
For users, social sharing is one blatant drawback of separate mobile sites. "Horrid" mobile experiences include slow mobile site redirects, and receiving the wrong version when the device isn't recognized.
You only have to look as far as their own websites for proof that Google sees the benefits of responsive. Most strikingly, Gordon describes how Google views responsive sites and mobile SEO. Google can attribute all of your search reputation and understanding of your site to a single URL, no matter what device its being used on.
Of course, Google prefers the mobile approach that's best for your users. But for those considering how Google will rank their site, the goal should be to increase Google's understanding of your content. This is an exceptionally strong signal from Google as to which mobile approach is best for SEO.