I don't think we have specific models yet, so it's hard to get exact numbers, but generally speaking, Haswell i5 and i7 are very close to identical on single threaded applications with the i7 doing 30-40% better on multithreaded benchmarks since it has hyper-threading (but as in everything multithreaded, your real world millage can and will vary wildly).
Also, I think there's a good chance that I/O is going to be your build bottleneck, not the CPU (though it's obviously depend on your project, language of choice and compiler), but in any case, the i5 is a capable CPU, you can do serious coding work just fine on it.
I'll wait until we have exact model information to make final judgment, but right now, as others have said, I think the 8GB i5 is the sweet spot and a pretty good value (would've been phenomenal value if the keyboard was included).