Some more detail peeping
iPad Air (1): When I tap on the screen it sounds (and maybe feels) a little "hollow", call it plasticky if you want, especially compared to the iPad Air 2.
BUT if I press on the screen (displaying a bright, high-contrast photo, text, or a checkerboard-like graphic) firmly, looking from an extreme side angle: The Air 1 seems more sturdy, nothing to see at all (by pressing way too hard there will be some "lcd waves" showing up—I won't make this very hard test too often, not recommended). On the Air 2 you can see from that viewing angle a little bagging, buckling/bulging even on slight pressures (on ANY place, even near the bezel). You can't see that when you watch the screen normally (from above).
Nothing groundbreaking, but it seems to be another "disadvantage" (not really…) for thinness.
When the screen is switched off (blank, black) and you watch against something bright mirroring on the glass surface (no need for an extreme angle), the bulging on pressure is almost identical on both iPads. So I'm guessing that the surface material is the same.
More comparing side by side: Brightness, colors and sharpness are exactly the same. The color temperature of the Air 2 is a tiny bit warmer altogether (this effect changed a bit with every new iPad—and I had them all). Viewing from side angles (landscape): Left/right: Air 2 a tiny bit better. Above/below: Air 1 a tiny bit better.