Rarely will you find the CPU in any of the new Macbook's to be a bottleneck. I have a 2012 Macbook Air that's done it all like a total champ: python web dev, rails web dev, iOS development, scala/akka projects, etc... For a while I was running a full rails stack and Xcode + the simulator to tag-team dev on the client/server. I'm still impressed that this tiny little featherweight computer has been able to keep up with me.

Definitely max out the RAM first. AFAIK it's not possible to do that anymore since it's soldered onto the mainboard. I have a (work machine) 15" retina that is a year old with 16GB of RAM that I can't really exhaust. My personal 2012 Air has 8GB and even that is pretty sufficient for most tasks, but these days I'd go 16 in a heartbeat.

Processor wise, go with whatever you can in your budget. The chip is pretty much the same whether you go for a lower or higher clockspeed, so there aren't any hidden benefits to the top-of-the-line one. It'll just crunch numbers faster.

i5 vs i7 is really dual-core vs quad-core. I wanna say that is physical cores, so in reality you're looking at 4 and 8 logical cores thanks to hyper threading. It really depends on your workload. An i5 will be a little better on your battery, but an i7 will help if you're doing a lot of multithreaded programming.