I think it depends on what features are most important to you. The big advantage of the Air is the A12 processor, it is a couple of generations newer and is faster than the 10.5" Pro. By contrast, the Pro has the 120Hz display, stereo speakers, etc. I'm not sure there is a single right answer.
In my case, I decided to wait. I've though about buying a 10.5" Pro, since they are currently on sale for the same price as the Air. Instead, though, I seem to recall the last iPad going on sale relatively soon after being released last year. My thinking is that we'll likely end up seeing sale prices on the Air -- and my hope is to find one $100 or so off, even if I have to wait until Black Friday/Christmas to see those prices.