Sigma 10-20 or Sigma20mm1.8 Need advice!!!!

Hi,

Trying to decide between the two lenses. The widest lens I own is the Nikon 24-85. Is it worth just a small drop to the 20 1.8 or would it be better to go for the 10-20. The 20 would have better advantage in low light. But is there that much difference between 20 and the 24 I already have? Advice, comments and any alternatives are welcome. Thanks