everywhere, pretty much everyday. So I would go lighter weight, 13 in or even 11 in. I had a 13 inch laptop.
As for MAC vs. PC, it depends on the major I think. Business majors and such use programs that require a PC. I did Biochemistry, and didn't really need my laptop for more than Microsoft Office and using the internet.
I started off with a Dell, but it crapped out during my Junior year, and I switched to a MAC and I don't regret it. If I had to get a PC, I would probably go with a HP or Acer. Whatever you do, invest a little more in it so you get a good quality laptop that will last. And make sure to buy Microsoft Office.