is it ok to use a 17' mbp charger on a 13' mbp or vice versa? what would happen if you do? My wife has a 13' and I have a 17'. I didn't realize there was a difference in the chargers so therefore I am sure they were switched and used on the wrong mbp more than once in the past 18 months! I have to buy a new one and wonder if one is ok for both?