3L here...
Congrats on deciding to study law! I traded in my macbook (white; 2006) when I first came to lawschool for a macbook pro 13. I found the pro to be adequate for class, but a bit heavy and distracting. The problem with apple computers w/ the new operating system is that it makes it almost too easy to "multi-task" in class. I cannot count the number of days I spent browsing around the internet on random blogs or just playing with the trackpad while trying to stay away during lecture. As a result, my attention and grades suffered a bit my 1L year. So I would keep that in mind when choosing a mac.
As for taking tests on a machine, you are going to want a device with at least a 13 inch screen. Legibility, typing speed, and screen real estate are incredibly important for softest exams, especially when they are open computer or "unlocked" exams. Remember, these tests are bell-curved, thus any potential advantage should be leveraged.
All in all, for lawyers, technology is not very important. You need a simple product that can essentially be a good typewriting and research machine. I cannot, however, stress how important the screen quality of your chosen product is. You are about to start using a computer a hell of a lot more than you are going to want to. In the past three years, I have probably spent over 1/2 my waking hours in front of a computer screen. As a result, I have become MUCH more concerned with the quality of the screen I have to stare at for hours on end. Screen resolution affects my quality of life, as sad as that sounds.
It was this concern that eventually led me to purchase an iPad Retina for note taking. I use the iPad in conjunction with a Bluetooth Keyboard and an App called Evernote for all my classes. I prefer the iPad to a computer due to its light weight, beautiful screen, and inability to multitask. Yeah, you heard that right: inability to multitask. Having to reach forward and interact with the screen is distracting for your fellow students, your professor, and yourself. As a result, I tend to keep multitasking during lecture at a minimum. My grades and class attention have really improved since purchasing this device.
And now to the recommendation. If I were you, I would go with the 13 inch Macbook Air, or if cash is not an issue, the new 13 Macbook Pro Retina. Despite it's cost, the Macbook Pro Retina will have an absolutely fantastic screen, and it might be worth the investment. The additional screen real estate will help give you that edge on exams, and will also be useful when researching legal issues. It really is all about the screen.
Of course, this is all coming from a guy who would buy a really luxurious car if he had to commute 3 hours a day in rushhour traffic. I like the little things.
Hope that helps.
-N