D800 & Sigma 10-20 or 17-35

Hi,

I have just brought a D800 and am really pleased. While I save up for a Nikon 16-37 I need to find a ultra wide angle lens to use.

With my old D300 I had a Sigma 10-20 HSM should I use this in DX mode or purchase a second hand Sigma 17-35 in full frame. Unless you can think of another cheap ultra wide angle lens.

I tend to take landscapes or interiors of churches. Mega pixels do no not matter to much to me. Sharpness is important.