I'll get the 16-35 and she can have my 17-40
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The 16-35mm doesn't review that well.
http://www.photozone.de/canon_eos_ff..._28_5d?start=2
Also $1700 is getting up there in cost. That's getting close to the Nikon 14-24mm F2.8 which is still king in this lens category (at least until Canon brings out a competitive wide angle 14-24 at some point but it would also be even more expensive more than likely). The Nikon 14-24 outperforms a Zeiss 21mm prime which is considered one of the best Zeiss's. My main checking point of these types of lenses is whether they show coma wide open, for suitability for nightscapes. Most of them seem to and that's not something easy to work around unless you plan to crop a lot and then why bother with the wider aperture.
Its heavy, it can't take filters without expensive adapters but it is one awesome piece of kit.
Adapters are plentiful. I have a nice Novoflex one that has aperture control and is very well made.
Or get a Zeiss 21mm with a Canon mount. The Tokina 11-16 seems to do well and is worth looking at.
I'd go the Nikon or the Zeiss.
Greg.