I would say that if they are making you stop to breath etc, that it is a case of irritable uterus and true contractions, rather than braxton hicks, I got these with my first pregnancy. It was not until my second pregnancy that I got braxton hick contractions and thought - oh they don't actually hurt they are just little tightenings of the muscles, that I realised how much different it was from the true contractions of irritable uterus.
But I don't think that there is any link between braxton hicks and true labour being shorter or longer. With my first pregnancy, despite having been having true contractions from 33 weeks (measured at hospital), he came at 39 weeks and it was a heck of a long painful back labour, the contractions before had done nothing really productive, whereas with my second pg, I had plenty of ok BH's and a long pre-labour stage, the actual labour was fast and furious, but a doddle really compared to the first!.
So I would suggest not worrying about the actual labour being worse, it may or not be, it is only time that will tell!!!!
Luckysalem - you may have a gloriously easy birth too, some people do get it all ways!!! I hope you are one of the fortunate ones.
Sarahjct - are you sure that they are BH, could it be irritable uterus??