17-40 f4 for lower light vs 16-35 f2.8

[EOBeav]

They both let in the same amount of light. The difference is that the 16-35 let's you go all the way out to f/2.8. If you're shooting landscapes (an ideal situation for the 17-40), you're rarely going to want that kind of DOF.

 

[wickidwombat]

The 16-35 f2.8L II is also very snappy in the low light AF performance on the 5d mk3 its superb in low light wide open