Looking forward to some head-to-head MTFs with the 16-35 f/2.8 GM (both wide-open and at equal apertures), now that the latter is shipping. They each play different roles, but it's hard to justify carrying both of them on the same trip (or even owning both), particularly if you're also carrying tilt-shifts.
The 16-35 is a stop faster, which is critical for certain subjects such as night landscapes with discrete stars. It also takes filters, although these are far less useful at ultra-wide angles than at narrower angles, since geometry leads to light forming the edges and corners of an image passing through a much greater thickness of filter than the centre (e.g. a 4-stop ND filter may well be 7 stops in the corners, due to light passing through a greater thickness of filter). However, if you're already carrying tilt-shifts, your use of the lens may be restricted to when you absolutely need the extra light (and can get sufficient DOF at f/2.
and for UWA action shots, since, between a 17mm and 24mm tilt-shift and either a 24-70 zoom, 35mm prime or 1.4x TC, you pretty much have the entire 16-35mm focal length range covered sufficiently.
The 12-24 is wider and lighter. However, subjects and compositions which lend themselves to such wide angles of view are usually amenable to stitching, which gives you an even sharper and higher-resolution image. If you're already carrying a 17mm or 24mm tilt-shift, shift-stitching is very easy, with minimal parallax error (or none at all, with the right accessories), giving you a horizontal angle of view equivalent to a 10.2mm lens (with a 17mm lens) or 14.4mm lens (with a 24mm tilt-shift). So the wider angle of view may not actually be that useful even when you do need the additional angle of view, except in rare circumstances (e.g. high dynamic range situations - stitching HDRs is a pain).
So, which one to get may well depend on which one is sharper, as opposed to differences in the focal length range or aperture - both of those things are situational and could lead to one lens or the other, but sharpness is almost universally a good thing.