I feel the 5S to 6 is a more substantial upgrade then the 4 to Air.
There Air got thinner and lighter, and thats nice but thats pretty much all there is too it. The Air is the 4's replacement.
The 6 while a new iPhone its different enough to still be sold next to the iPhone 5S. It offers new technologies (to the iPhone) like NFC, improvement to the camera, barometer, plus what I've been waiting for...a larger screen.
iPad upgrades have never impressed me, I went from an iPad 2 to 3 to Air then back to the 3. And if I had to do it over again I would have stayed with the iPad 2. I'll buy another one when it does something more useful for me which will probably be a very long time.