First, see the megahertz myth - you can't compare processors from four years apart based solely on clock speed.
Either machine would be great for iOS development. A couple things to note:
- I use a 2012 13" Pro as a daily driver. I put a Crucial MX300 SSD in it and it's a great machine.
- You'll be able to buy the 2012 for significantly cheaper, and it'll hold value slightly better (all Macs hold their value immensely well).
- You need to be able to run the latest version of macOS for iOS development. Based on patterns, I suspect the 2012 Pro has 2-3 years of support left. A 2016 machine likely has 6-7.
If you get the 2012 Pro, buy an SSD along with it. It makes a huge difference compared to a hard drive.
Overall, I think it comes down to your specific situation. If your company is buying a laptop for you, go get the 2016 Air - then you don't have to open it up, and they get a laptop that's useful for a longer period of time.
If you're using your personal funds, I'd probably lean toward a 2012 Pro. If done properly, you should be able to sell it in a couple of years at a minimal loss. I bought mine in 2014 for $800, and could still get ~$600 easily. $200 + $99 for the SSD isn't bad at all for three years of solid use.