Just adding my two cents. I started out with a heavy aluminium-framed road bike, upgraded two years later to a 2kg lighter aluminium bike, then two years later upgraded to carbon (losing another 2kg again). if your wife's a confident cyclist then she may be fine going straight from a mountain bike to carbon. I personally would have found it too much of an adjustment. My carbon bike rides completely differently to my last two, and it's taken me more than I'd realised to get used to it. I love it though, don't get me wrong!