I was torn on this exact same issue, I ended up going with the iPad.
Coupled with a Bluetooth KB, I don't think there is much difference in using them to take notes. In fact, the iPad may have a slight advantage since you can easily draw graphs and diagrams, plus some of the note taking apps are really good. Also, because the KB is a modular component its still more compact than a MBA when you need it to be.
What really pushed me to get the iPad was the price. The iPad leaves me $500 richer, and there is a lot I could do with that money. Heck, if the iPad doesn't work well, I could keep it and still a buy a Win7 Laptop with the cash I saved (I actually like Win7).
The biggest potential downfall of the iPad is that some online course material is still tied to flash, so thats something you definitely want to look into.