It is true that many use Mac and most others use Linux. There are a few who do use windows and the reason this works is because no one uses their laptop to do science. They use it for writing slides for the talks that they have to give every week to their collegues. Real work is done on a computing cluster. Any operating system which has ssh will work for HEP. I know I had a windows, linux and osx laptop at various points. It doesn't really matter that much because as I said you are going to do all of your real scientific work on the cluster at the lab. I always liked to have a small, light-weight laptop because it was less onerous to carry around the lab, to conferences and collaboration meetings. What you really want is battery life so that you can sit in the back of a boring meeting and write code on the cluster in your ssh terminal. I think I liked my macbook air best, but had a lenovo x200 as well that worked great because it seriously had ~10 hr battery life.
Just my two cents. Battery life was the most important feature because all real work took place on the cluster.