What you consider an ultra-wide lens is a bit different than what I would normally consider an ultra-wide lens. 24mm is a very, very nice focal length for a wide angle lens, but I would not consider it to be an ultra-wide and I most certainly would not consider a 28mm lens to be an ultra-wide angle lens.
I have a 17-35mm f2.8 which I like a great deal on film and a 10-24 DX which pretty much duplicates it except for the speed. You start getting dramatic changes in perspective once you go below 24mm. If that is what you want, I would agree with sevencrossing's recommendation to get a 16-35mm f4 VR.
One other thing, the depth of field of ultra-wide angle lenses is typically quite great, especially when stopped down and is the hallmark of their classic use. That is very, very different from the idea of shooting an f1.4 prime wide open. Neither approach is better or worse, just different. You should consider that carefully before making your selection.
Cheers!
