I think I might pick up the Sigma 1.4x TC as well at the same time to give me a little more reach on my long end as well
I just got the 10-20mm and the 1.4 TC. The 10-20mm is a great lens. I love the HSM. The lens is pretty sharp at all ranges. I haven't used the 12-24. But I say go for the 10-20. I've been very happy with my purchase.