Well I have a laptop but its too big to bring to school an it's like 7 lbsThe MacBook Air. If you have no other computer, having a MacBook Air is the smart choice. If you can have the Air AND the iPad, even better.
I guess if you have the money, the best thing to do is to get a fully-functional Macbook Pro (or Air 13inch) and an iPad.
I was trying to use a 11inch Air as replacement for iPad2. Does not work. For starters, the 3-4h battery life is very annoying, you cannot get much done in that time. And reading book on that 11inch horizontal screen is a pain. Specification of the Air sounds much better than an iPad, but it does not transfer to better user experience.
So... why not try both and see which one fits your tasks?
For example reading books or watching iUniversity during flight for bus trip. My Air always gives up in the middle of it. Or taking note for an entire day of classes, only iPad can do that without charging. You know class rooms, not every one get a plug.What do you do that takes longer than 4 hours, which is away from a plug? And can be done just as good on an iPad?