If you're going to be carrying books, a laptop as well as clothes, I'd say go for the pannier as well. I commute with my messenger bag but usually I just have a sweatshirt, my lunch, my U-lock and some other random small things in it. Whenever I have my laptop, a large book, or anything large and stiff in it, I end up getting a bruise or a sore spot on my back where the hard surface bumps against it when I ride.