You're going to buy a new Mac, and you've narrowed your choice to one of Apple's new sleek, speedy MacBook Pros with Retina display. Seems like an easy choice, doesn't it? Just decide which screen size is right for you: 13-inch or 15-inch, then pull the trigger. Not so fast. There are other considerations you should make, as well. Because screen size isn't the only different between the two machines. This guide should help you iron out some of them.