I've read different threads but never got to a clear conclusion, what are the differences, advantages and disadvantages of using user-scalable=no on case A or case B?
I'm going to do a responsive website, which case is better then?
CASE A:
<meta name="viewport" content="width=device-width, user-scalable=no">
CASE B:
<meta name="viewport" content="width=device-width">
You don't need to use user-scalable=no for responsive web design. You use it if you want your web app to feel more like a native app (in terms of zooming). You can control that users wont break your design if they zoom in. But technically, if you're doing responsive and your design breaks when zooming in, then you're not doing it right.
But, if you absolutely need to use it: "... however keep in mind that zooming is an important accessibility feature that is used by a lot of people. Turning it off should therefore only happen when really necessary, such as for certain types of games and applications." http://dev.opera.com/articles/view/an-introduction-to-meta-viewport-and-viewport/