The first place to start your decision, as always, is to consider your use case carefully. The mythical iPad Air 3, which I imagine will appear in March, will be a poor note taking alternative unless Apple is ready to bring the pen to the Air design, and I would guess that is at least a year away. Many people I know say they wouldn't even consider the iPad XL if there were an Air with the Apple Pen. So, if a pen and note taking are important, score one for Surface Pro 4.
From my personal experience, my iPad Air got little/no use when I got my Surface Pro 3, and even less since upgrading to the Surface Pro 4. Other than some neat games on the iPad, there is literally nothing I can't do with my SP4. The only other factor is battery life - iPad Air has the SP4 beat hands down on that front, UNTIL I stop to realize what I'm driving with that SP4, then I don't mind losing 3 hours (I almost always get 6-7 hours on my SP4 when doing iPad Air "stuff" but about 5 hours when running productivity applications).