well when you mention the 5k for the k20, that's most likely the price for a k20a with lsd trans and wiring harness and shifter box, cables and ecu. and when you say 600 for the k24a4, that's just going to be the price for the longblock, meaning no trans, and quite possible nothing else. so you are still going to have to get a trans, and ecu and shifter box and cables, and once you add up the price for the entire swap, whichever motor you choose, you're going to end up spending anywhere from 4k to 8k. depending on the parts you choose to buy. the k24a4 are low compression k series and people really only use them if they are going to build the motor. basically it will be equivalent to a gsr swap. the k20a on the other hand is the higher compression higher revving motor that performs very well with just bolt ons and kpro. so if those are your only 2 choices, and you are planning on putting the motor in and leaving it stock, definitely choose the k20.