Xcode 6&7 performance on Macbook Air 2012-mid
I use both Xcode 6 and the 7 Beta on a mid-2009 MacBook Pro with a 2.66GHz Intel Core Duo, 4 GB of RAM, and a 260 GB Intel 530 Series SSD. It's good enough for me to develop apps on, though it's not like I'm using Metal to make 3D games or anything. I personally wouldn't get a MacBook Air, as it's just not a performance laptop and you'd get a lot more out of a slightly older MacBook Pro. The one I have is pretty good for what I use it for: Xcode, Final Cut Pro X, web-browsing, etc. But that's not to say I wouldn't want a newer MacBook Pro.