Apple - Xcode Quad Core Optimization
Xcode will make good use of many cores - especially when running the simulator, unit testing at build time and just coding / compiling / searching documentation.
Also check to see if the processor with more cores has a higher speed boost when only one core is running. The rated speeds are controlled more by thermal conditions under full or benchmark loads and can run faster when the current workload is single thread dominated.
In general, the IO is the first thing to slow down a developer doing multiple normal dev tasks. Next is insufficient RAM and a distant third underpowered CPU. Do take your Xcode installer and some sample projects into an apple store and ask if you could run some tests on an air as well as larger machines. Some specific code bases and settings might be more polarized than the general case and more biased to CPU with larger cache or speeds, but the smallest air is so much faster than my three year old 15 MBP it's not very funny at all.