reliability.. you can impot a rb and not have to crack it open(aside from the timing belt and oil pump)the z also but remember i-6 has less problems than a v6 and since its a single turbo less parts to buy and more space to work under the hood.. and screw f&f this is the real world but the ability to crank your car up to 800hp for dyno duty then detune for 500 for daily driving is golden.. and the avalibility of the apexi power fc(half the price of a aem) makes the rb more appealing..