I have asked myself the same question as I would like to upgrade my machine. I currently run a MBA 2011 with 4GB of RAM. It is by far my best computer purchase. When I'm at a desk, it's plugged into an external monitor. Until Mountain Lion, I never had a problem with RAM, but I've adjusted how I do things on my laptop to fix the issue. I wrote up a PrOACT and found that it came down to 1 point for me.
- Do I require the dedicated video card and is that video card worth $1000+ increased bill
So if you're only looking to do development, go for the MBA since it's better cost for use. The Retina doesn't matter if you're always plugged in like I am.
Your only question you should ask yourself is, do you either want to use multiple monitors which the MBA can not do or do you want to be able to play games that require the video card?
Latency to me is not a problem. I live in asia and there is internet everywhere. Would it really make that much of a difference for you?
8GB of ram + SSD will get you far!