Are both 32GB? If storage sizes are the same, it's pretty much a coin toss.
Air 2 has the nicer screen with less reflections (easier for bright light conditions). It's also a bigger fingerprint magnet and has greater friction compared to Air 1 screen (which I expect is what the 2017 iPad has).
Performance-wise, 2017 iPad is likely faster (extrapolating from iPhone 6s/6s+ benchmarks) but the Air 2 feels like (and is) the more premium device. In terms of longevity? The 2017 iPad has a slight edge.
Personally, got the Air 2 128GB ($399) for myself. The screen is more important to me plus it was $30 cheaper than the 2017 iPad 128GB.
I am getting my dad the 2017 iPad 32GB ($329) though. Partly because the $299 Air 2 32GB was already out of stock and partly because I want to play with and run benchmarks on the new model.