Sigh, I'll explain it in layman's terms for those that are a bit simpler in the head:
There is more to than just being "stronger". Just because you're physically adept doesn't necessarily make you a better fighter. Law is (from what I've seen) not stronger than Luffy, but his ability can easily incapacite Luffy if given a proper opportunity considering how large he can make the "Room". Luffy would win an obvious 1 v 1 fist fight, but Law has a decent chance of winning due his broken powers (splitting up bodies, manipulating objects around him, etc). CC, like Magellan, can also easily incapacitate his enemies. If he were more capable, he could have dodged Luffy's attacks simply by making holes in his body (he's gas after all) as well as manipulate the oxygen around him, utilize poison, etc…and basically slow Luffy down. However, obviously in a fist fight, CC cannot win which is a given.
Although i do not completely agree with Foxy, i still would like to explain what he means with "stronger" since you did not seem to understand him. For Foxy, the term "stronger" does not only refer to physically stronger but a superior, overall fighting capability.
In my opinion, you fail to acknowledge Luffy´s feats and capabilities in your estimation how a fight between Luffy and Law would turn out but you pretty much concentrate on Law´s impressive abilities. While Law´s DF is extremely dangerous and gives him the possibility to end a fight in an instant, he still would have to land a hit on Luffy first which would not be an easy deed considering the speed Luffy has shown on PH ( http://www.batoto.net/read/_/106234/one-piece_ch670_by_mangarule/6 , http://www.batoto.net/read/_/107517/one-piece_ch671_by_mangarule/11). We also have seen Law having problems with faster opponents (http://www.batoto.net/read/_/134258/one-piece_v67_ch662_by_mangarule/7 , http://www.batoto.net/read/_/134827/one-piece_ch683_by_mangarule/15). So, like darkrunnerjay said, the fight could go both ways which, for me, puts Law and Luffy on the same level. I also would not be surprised if Kidd is also one the same level as the other two, considering that Oda has introduced Law and Kidd as Luffy´s rivals. I doubt that Oda will drop that rivalry, at least for the beginning of the NW.