Buy the ethernet cable. There is wireless internet coverage all across campus except in a a select couple of dorms, I can't remember which. The wireless is pretty good, but an ethernet cable is only $15-20 at your local Staples or online and wired internet is almost always faster and more reliable. If you get a 25' length cord you can comfortably use it anywhere in your room.
Check to see if your dorm has wifi in the entire building. If not, then the ethernet cable will be required anyway. Also, if you don't want to buy it now, you can wait until you move in, try the internet out for a while, and then buy one and have it shipped to your dorm, not a big deal.