New macbook air or late 2012 retina macbook pro

The 2013 13-inch Retina macBook pro was discontinued in July 2014. Per Apple's published service policies, parts and service support for that model stopped in June 2019 in most places, so Apple won't work on it any more. Maybe an Apple Authorized Service Provider who can get parts, but that is not a "sure thing."

The 2018 MBA is also slightly faster than a 2013 MBP per GeekBench scores recorded in the MacTracker database:

Like the others, I say go with the MBA and save yourself a lot of grief.

I'd never purchase a 7 year old computer without knowing how it was used. For example, my wife slings her notebook into her briefcase and throws the briefcase into the backseat of her car twice a day and she pounds on the keys when she types. Her notebooks have never survived 5 years without at least 1 repair. My brother on the other hand is OCD and treats his computers like they are precious fragile jewels. I'd not hesitate to buy a used computer from my brother. My wife? No way!

You can check benchmark results to get an idea of which is faster - I honestly don't know. But unless you are writing huge apps I don't think you are going to notice a big difference in compile/development times.

You'll get more mileage out of the MacBook Air because it will not be obsolete in a couple of years like the 2013 MBP. That means you'll be able to upgrade the OS a lot longer, install important security updates and obtain parts if needed.