Sigma 15-30mm or Sigma 12-24mm for 1D MarkIII ?

Hi,

I recently got my new ultra wide lens Sigma 15-30mm with a very good price 275 Euro from eBay, I took some photos it looks nice, the issue is, this is not designed for FF or Mark III body, my question is would it be wiser to pay little more and upgrade it to Sigma 12-24mm as its the only one from Sigma for FF cameras?

Would I notice any big difference in the 3mm range and what about the image quality is it better?

Need your advise

Regards