Buy 1/2 a gallon of milk, and a pound of coffee or chocolate powder. Take your backpack and carry it around with just the milk and then with milk + chocolate powder. That should give you an idea.
When you're done, heat the milk, put chocolate powder in, and enjoy a hot chocolate!
The other problem is the size difference. I find the 15'' easier to handle than the 17'' model, while the weight difference is less noticeable. For this you will need some cardboard and the specs from the apple website, to build realistic mockups of the 13'' and the 15'' models.