The baseline MBA should do you just fine. Unless you are planning on doing development on 3D games with physics, 4GB of RAM will be more than enough for the next 4-5 years. Xcode will be fine. It may take a little longer to compile, but unless you have 100+ pages of code, 8GB of RAM will only cut a second or two off your compile time. Especially considering you can pick up a baseline MBA for as cheap as $750 as a stock model, $1050 for 8GB of RAM is not worth the extra $300.
The one thing I cannot stress enough is to not get a cMBP. It will cost you about $1400 to even get it close to the performance of a baseline MBA that costs $750. On top of that, it will weigh 1.5 pounds more, have a worse screen, about 1/3 the battery life, worse graphics, and produce more noise/heat. Back in 2012, I would have recommended it to maybe 5% of people, but the price has not dropped, and other computers have adapted new technology at a lower price. There is absolutely no justifiable reason to get a cMBP at this point in time. The only benefits are an upgradeable HDD (which is on SATA, not the current PCIE), upgradeable RAM, and an optical drive. You can pick up an external optical drive for $20 if you ever need to use a CD/DVD. RAM is really the only reason to get it, but even then, for $1400 you might as well get the 8GB/256GB 13" rMBP for the same price (assuming you upgrade the HDD and the RAM on the cMBP).
Best,
Matt