Peanut butter first.
You get the peanut butter on the knife first, otherwise you may get some jam in the peanut butter jar. If that happens, it is more likely to go bad because the jam in the peanut butter will go unrefrigerated. Any peanut butter in the jam jar will be fine because it won't go bad before the jam does.
Otherwise there is no point to putting either first, at least the way I make my sandwiches (which is to say, peanut butter on one slice of bread, jam on the other.)