Which size fork?

I recently purchased a 2009 Trance x4 frame to start building slowly. I thought one of the first things I would purchase would be a fork. I noticed that the x4 came with a 120mm fork and the x1 and x0 came with a 140mm fork. I was wondering if I should go with the 140mm instead of 120mm? I am newer to mountain biking and not sure if I would notice the difference. I am open for suggestions. I will probably try to find a used fork in either size.