That you would argue against shooting the 300 f2.8 VR plus TC-17E on 24 MP DX, but subscribe to shooting the 500 f4 plus TC-14E on 36 MP FX. In terms of reach, there's little to choose: 1050mm 35mm eqivalent at 12 MP in the first case, 1190 mm in the second. My argument is that you'd get the occasional shot at less than 5m. - especially using calls - and these shots might be very high quality. You'd lose very little at the long end. And optically both are very good.