You can use this program to track how much data each computer uses. This will mostly be useful if you guys are doing your downloading on separate computers.http://download.cnet.com/BitMeter-2/3000-2381_4-10398686.html
Honestly though, the BEST solution stop arguing about who uses more internet. You pay for a connection speed. Not for an amount downloaded. Therefore you are both using it and should both pay equally.