Ken Wisnefski lends his 2015 SEO insights to the Leftmark blog

“Something ecommerce companies can no longer afford put off is making sure their web-site is ‘Mobile-Friendly.’ Google announced at the end of February 2015, that beginning on April 21st , 2015, sites that do not adhere to Google’s Mobile-Friendly criteria will see a lower rank, meaning if your site was only designed for desktop computers and presents mobile users with things like poorly formatted text and links, slow load times and horizontal scrolling, then your site will rank lower in Google searches done on mobile devices, and you will lose traffic and business.
The question then becomes, should you employ adaptive or responsive web design? Adaptive is more rigid and uses predefined trigger points to fit different sized screens, while responsive is driven by a set of rules based upon screen size and percentages.
Right now, I would recommend responsive, because by making everything dynamic, you’re taking into account devices and screen sizes that don’t even exist yet.”