I doubt a shallow DOF is really much of an option on a 24mm anyway, at least in any useable terms. The extra stop is nice, but not worth the money unless money is not an issue. I was just reading your thread on the blad dilema, so I assume you are not sitting on a million dollar disposable income - in which case the choice is obvious.
The only real argument I can make for the f2 is that in the same conditions that call for a 2.8 app, the f2 will be stopped down a bit, where as the 2.8 wide open.
If you are thinking landscapes or anything outside, I would say 2.8, especially on a short little lens that you can hand hold to 1/30 anyway, and could probably squeeze a 1/15 with something to lean on - not to mention that landscapes without a good tripod are a bit of a problem.
If you are going to do some realy funky indoor, available light stuff, well, that f2 might come in handy... but how often?
I had the same issue with a 28mm Canon lens that I was thinking of buying recently...f2 vsf2.8... and I know one stopis twice the light... but does it justify almost three times the money? For me, in my current situation - no. Tobe honest though, there is something neat about owning a special piece of equipment, just a bit better, just a bit more rare... perhaps its shallow, snobbish, etc... but I know that my 80-200 "L" series lens is my baby and I sometimes take it out of the case just to play with it... although I shoot my 50mm 90% of the time! Yes, I am weak little human