Well all I can say is having owned both Giant and Specialized carbon frames, both seem to be of good quality. I get the feeling there is very little in it in terms of carbon frame quality between all the well known big brands. I would be more wary of buying a carbon frame from one of the smaller boutique brands, who may have little control over the manufacturing process and probably little experience anyway of carbon design/production. There are exceptions of course. For example Factor bikes in the UK, who gained their carbon experience directly from the motorsport industry.