I vote for the improved 16-35mm. Less lens switching, easier filter situation. All-in-all, more practical for weddings and real-estate (interiors).
I'm still enjoying my eight year old 16-35mm version one, but if Canon (or Sigma!) could come up with an ultra-sharp from edge to edge version, especially one with significantly less distortion at the 16mm end, I'm an upgrader.
I do NOT want a $2000+ lens with a bulbous front-element that cannot be protected by a UV filter, thank you.