calling responsive web design a trend makes it sound as if its something thats a passing design fad, simply its not, a few years back you could have gotten away with a static sized site how ever most likely you’d have built your design around either the common average desktop resolution or some lower common resolution and as resolutions only tended to get bigger, you’d be safe. But then mobile devices came along and basically threw the rules out the window. Not catering for mobile devices these days is kinda like deciding to ignore the design quirks one of the major 5 browser’s.
As for a mobile site vs responsive design, responsive wins out because a) you don’t have to maintain and update two sites b) some of the big player’s have come down on the side of preference of responsive design.
Having said that, responsive design does take more time than simply a static site, far more testing tweaking and lateral thinking, idealy it should be part of your standard package it should be the norm for designers to consider mobile devices now just look at how many smart phones you see when you walk down the street or look at brwoser statstics. But its going to take some time for that to completely sink in across the community and ofcourse its going to cost a bit more, my opinion is that it should be part of your standard sales pitch but give the client an opt out rather than an opt in, and explain to them the advantages of having a responsive freindly site.