I bought the auto some years after the 642 and I find the auto easier to shoot but after carrying both I find the slight weight difference to be quite noticeable, the 642 seems much lighter than the LC.
It's pretty much a certainly that you'll carry more then you will shoot. The lighter and more comfortable the gun is to carry, the less chance you'll leave in in the truck or home, defeating the purpose of the CCW.
My opinion on the subject is choose the one that easier to carry, THEN, practice with it until you can shoot it as good or better then your heavier gun you now shoot better.
Any gun is useless if its not available when you need it.