not sure, but i think the 12-24 was out first... they followed the Nikon 12-24, which was the first ultra wide for APS-C sensor.Well, considering Tokina put in extra time to develop/test it (released a few months later than the Tamron), it might perform better.
Just a little bit of OT, was the Tokina 12-24 also released later than the Tamron 11-18?