what is your goal engine wise? do you want an engine that revs to 7500 - 7800 rpm reliably (K24) or do you want an engine that can rev 8000 - 8800 rpm reliably (K20). The K20 was designed to operate at higher engine speeds more or less used for road course type performance/circuit driving. The K24 was designed for a touring type engine, loads of low end TQ and little top-end performance.
Even with the K20 head on the K24, the K24 will still be a 87x99 engine. If you are not going to do cams on the K20/K24 there really is not point to doing the frankbuild. Stock cams just keep the K24 as is. This topic has been cover thousands of times, and still the best answer is how do you plan on driving the engine and what do you expect from it ? Do you want to be able to rev match easier, multiple downshifts from highspeed braking to enter turns, do you want an engine that prefers to be treated like a bitch and thrown around ruff - get the K20. If you do straight line racing, K24. If you want to rev, K20.