I don't have either of those devices, but I figured I could offer some basic data points anyway. I put my iPhone 6 Plus and iPad Air side by side and recorded a bit from a Wikipedia article on Mary Had a Little Lamb. I figured it would be slightly more interesting than reading the poem itself.
I used the Ferrite app with the default settings and no adjustments/editing after. The devices were sitting side by side and recording at the same time.This is about as fair as I can get with my setup.
The iPhone is a model newer than yours, and the iPad a model older than what you want, but it should give you a rough idea. If one or both of them are better than what you are getting on your iPhone 5s, then you can figure the iPad Air 2 is boing to be a bit better. Hold in mind Apple hasn't done a lot with the microphones on their devices. Not enough that they feel like bragging about it anyway.
I apologize in advance for the lousy reading. Obviously no one is going to hire me to be the voice of any audiobooks.
Hm, the forum won't handle audio files, so here are some links from my Box account. I use it instead of DropBox when I want to leave stuff around indefinitely. I have a lot more space available on Box.
iPad Air version:
https://app.box.com/s/1slegelvccyyvejib0un52jrkgn2ok32
iPhone 6 Plus version:
https://app.box.com/s/ez7be9stmbj4gwy0c2sc3c70dgqmsjo9
Note: Ferrite is free (with in app purchases). If you want to step up your recording game with some input/gain controls and some basic audio editing tools, this is a good choice. I've only used it a couple times (without any of the extra in-app purchased features) and like it. It's also getting some love from a podcast or two. If you are considering making comparisons, I suggest you get it and use the default settings. Differences in apps can make as much difference as the device.
