I am considering one or the other mainly for landscapes, but it could be nice to have f2.8 for the occasional indoor scene, there is a Canon cashback offer in the UK on the 16-35, still more than I would like to pay, but I have learnt in photography you have to pay for quality gear, my other concern is do these lenses vignette at wide angle especially with filters ( Cokin p system ) I would be prepared to upgrade filters over time.
Alan
Alan