Could be. However the Canon rep who let me test both of them, the working AF sensitivity of the 5DMkIII is 0EV. And I was able to test it too. The rep darkened the room almost completely and let me fire some test shots with both of them alternately, using the other body as subject on the table. The 5D MkIII was not able to focus in completely dimmed light, the 1Dx focused lightning fast. Only with the lights turned up a bit, the 5DMkIII was able to focus after some attempts. As of then, I immediately signed the pre-order
I believe the 5D mkiii also has -2 EV sensitivity for focusing so should be as good as the 1dx to focus in low light, maybe not as fast though.
Humm...interesting... The spec list does state -2EV:
AF Working Range
EV -2 - 18 (at 73°F/23°C and ISO 100)
But maybe it only works for ISO 100 which is not that useful in low light!!! I am not surprised though that the 1DX would focus better and faster in low light...afterall, it does cost almost double!
1DX, 24mm f1.4L II, 50mm f1.2L, 85mm f1.2L II, 135mm f2L, 24-70mm f2.8L II, 70-200mm f2.8L IS II : D800, D4, 24 1.4G, 35 1.4G, 50 1.4G, 85 1.4G, 24-70 2.8, 70-200 2.8 VR II