Google Announces Mobile Friendliness is an important SEO factor

With the announcement by Google that mobile friendliness will become an SEO factor, many eCommerce site owners are wondering what is their best option?
1.  Responsive
A responsive design will respond to the size of the browser window.  Shrinking the window causes content to drop down and to the left.   In a simple three columns layout of left, center and right, the end result is one column with basically three boxes containing left, center, right.
Knowing this,  a responsive design has to carefully consider what that end result will look like.   To develop a responsive design you need to design for desktop, tablet (optional) and mobile views so the programmer can see how the design will respond.  Some important design elements such as the main menu navigation which will eventually transition from a normal menu to a mobile hamburger icon.
2. Adaptive
Adaptive is a variation of responsive where, in responding, some design elements will respond, some will disappear and some will adapt, much like that hamburger icon, to look completely different.  To develop an adaptive design,  you need to design for desktop, tablet and mobile views so the programmer can see how the design will respond and/or adapt.
3. Desktop + Mobile
For existing sites which have a desktop only design, probably the best option is to make sure the desktop version works on tablet and develop a phone specific design. Just make sure  you tell search engines that the mobile pages are “alternatives” to the desktop page.
But what in terms of cost?
Adaptive is probably the most expensive route as not only does it require three designs (desktop, tablet, mobile) but it can often require separate front end code increasing the overall cost of design and support.
Responsive comes after Adaptive in terms of cost to develop.  It can requires less additional cost but the cost to integrate a responsive design can be up to 3 times more than traditional desktop.
Separate mobile is most the least expensive investment route.  Ideally it can be implement using the same desktop code base with little need to custom code.  However many sites opt for separate front end code as the least expensive implementation.  of course there’re are increased support codes as now you have to implement functional changes twice!