I'd go off LMP if you were sure of your cycle being pretty regular. U/S have a margin of error that gets bigger and bigger the later on you get as each babies individual characteristics start to come out. I was sure of my dates - my scan at 20 weeks showed both a) the head was 22 weeks and b) the leg length was about 18.5 weeks. So each bub is different. Turned out we just had a one with a big head and short legs...