There is also a small aperture difference, iPhone 6 is f/2.2, iPad Air 2 is f/2.4, roughly about a quarter-stop. This will result in slight advantage in low light and depth of field focus. It also means that shutter needs to stay open for a shorter time in the same light conditions, introducing less chance of blur. That said, I seriously doubt if any noon-professional could tell the difference in picture quality between an f/2.4 and f/2.2 lens.
And to answer the one question, the iPhone 5 has a 1.4 micron pixel size (f/2.4), the 5s is 1.5 (and f/2.2), and the iPad Air 2 is 1.12 - smaller. The 1.5 will gather more light in the typical picture, again meaning shutter is open less time, less chance for blur, better low light picture, etc.
So the biggest camera specs are definitely better for the 6 vs. the iPad Air 2. There is so much more that goes into picture quality though - software, hardware, signal processor, etc. I thought as an amateur, the 5s camera was great, the 6 will probably prove better, and I bet the iPad Air 2 will be considered excellent as well.