Those are almost exactly my use cases too; my difference is that I don't watch video but I do take my notes on the ipad, writing with the pencil. This works great because documents seamlessly work between the two devices.

By far I use the MBA the most, but the ipad is much better for a small number of use cases that are worth my spending on. And for those use cases the ipad air is sufficient, especially as I can now get a larger one. That's similar to the macbook air: I'm clearly a "pro" or "power user" and it has more than sufficient oomph (note: when I type 'make' the compilation doesn't happen locally)