hi mate..i have that sigma 10-20 lens and i have to say its not too bad..i have to agree with the low f range and the bad quality in low light but wide angles are usually used mostly for scenery outdoor photography and for the price its quite reasonable..love the autofocus on it too..this was mostly the reason i didnt get the tokina..and the nikon 12-24 was just ridiculously expensive..nearly 1.5times more..at the end of the day it depends on how long you plan to use your current camera i guess..
You're born, you take shit. You get out in the world, you take more shit. You climb a little higher, you take less shit. Till one day you're up in the rarefied atmosphere and you've forgotten what shit even looks like. Welcome to the layer cake son.