I purchased the 12-24mm Tokina Mk-I when I was shooting with the 24-70L as my go-to, mid-range zoom. Additionally, the the time, Tokina had not released the 11-16mm. I absolutely LOVE my Tokina.
However, now that I am shooting with a 17-55mm f/2.8 IS lens as my go-to, mid-range zoom; the 11-16mm f/2.8 would be a better match. Additionally, the f/2.8 aperture would give me some additional flexibility.
HOWEVER, I absolutely love my present Tokina (I have experienced no major flare problems) and, although the f/4 is a stop slower and the f/2.8 is 1mm wider, the 12-24mm focal range does have an advantage.
I do like the 24mm side because the 38.4mm equivalent is almost a wide normal lens. On a forthcoming trip to China; I will bring my 30D and 40D cameras with 17-55mm f/2.8 IS and 70-200mm f/4L IS lenses as well as my 12-24mm Tokina. If the 17-55mm should crap out for some unexpected reason; I could limp along with my 12-24mm and 70-200mm lenses. I would not be comfortable shooting with a 11-16mm and 70-200mm.
Additonally, for a photographer who shoots with a single camera, the 12-24mm lens would probably lessen lens switching in the field.
That said, in view of my present equipment, if I did not own the 12-24mm and was considering a wide lens, I would definitely consider the 11-16mm because of the f/2.8 aperture.