I switch, depending on the task. For math, I always use pencil, because I would feel dirty if I did it in pen. I my other classes, I'll usually use pen for notes, pencil for classwork and homework – The reasoning behind this is that I write much sloppier in pen, but also much faster and easier, so that's why I take notes in pen, then do work that my teachers grade in pencil so that it's legible and there aren't any scribbles.
For my pencil, I use a Pentel P207, 0.7mm (0.5mm is way too damn thin and stupidly hard to erase, especially for someone who writes dark as I do). My pen is a nice ballpoint that my dad got for free from Continental Graphics. It's quite heavy and the grip is pretty wide, but I like it like that.