Sigma 12-24 or Sigma 15-30?

I'm planning to get a UWA lens for a D600. I came from a Sigma 10-20 on DX, and altough I now the 10-20 equivalent would be the 15-30, I'm tempted by de wider 12-24 sigma. Given that my budget is little more than 500€ (is a lens a don't use that much), and the 15-30 is cheaper, which one would you recommend?