It depends on the classroom. Most of the lecture halls use the tablet arm (where the little desk folds up from the side of the seat). In other cases, you’ll be in a classroom, where you will have a rather large table with ample space for a computer and a notebook. I think most students would agree that you will always have space for either a computer or a notebook on your desk, sometimes both.
I think that you should base your decision more on whether or not your current computer is powerful enough for RPI standards. Unless you’re planning on having two laptops (which isn’t exactly rare), you should make sure that the 15” laptop has the power you need. Take a look at the specs for the laptop that students joining the program in Fall 2010 received: http://www.rpi.edu/laptops/laptops10/specs10.html.
Don’t worry about it fitting on the desk. My class has 15.4” displays and it fits well.