I've decided to buy a Sigma 15mm fisheye lens, but when I go to B&H, I find several different versions, one designated "DG" and others that aren't. Does it make any difference. Does the "diagaonal" in the title make a difference?

just make sure you buy it with Canon mount :-)

I didn't know tehre is a new "DG" version available. But Sigma "upgraded" many lenses recently into DG versions, or just renamed some of them?

All 15mm versions are "diagonal", different to the 8mm fisheye which is circular.

Julian