Hard to answer with the information provided.
30mm is considerably wider, but I found it perfect on a crop for creamy bokeh and thin DoF, this was with Sigma 30 f/1.4 EX DC which is a crop-only lens. Is that the one you're asking about?
On FF, 50mm would be similar to 30mm on the crop, so I'd go for that for a similar field of view. Although the new Sigma 35 f/1.4 looks really tempting from all the images I've seen!
Last edited by ddk632; 01-04-2013 at 09:16 AM.