I faced the same choice some three months ago.
I visited a Sigma dealer to try out the 30mm f/2 on my camera (7D but I also have a 40D). In a nutshell: I liked the quality of the photos very much WHEN THEY WERE SHARP. Sigma's AF struggles on Canon bodies -- it is OK-ish in bright light but can get really bad otherwise. With most Canon lenses, on peripheral AF sensors, when there's not so much light, at least on xxD / 7D -- it takes time and I need to press the AF-ON button a few times to get focus but I usually do get it. With Sigma, I feel it's worse, there's times when it never focuses correctly (it misses by a tiny bit but it is too much for my purposes, I need the lens to make photos of a newborn baby, eyes must be tack sharp)
I got the 35mm f/2 instead and I am very happy with it. I also very much like the close focusing distance -- again, very useful for newborn closeups.
AF on the 35mm f/2 is loud and not fast (not really slow, but slower than that of 85mm f/1.8 or 400mm f/5.6) but precise.
I really could use Sigma's extra 5mm width so I had also considered the 28mm f/1.8 but ultimately decided against it. I felt the 35mm f/2 had marginally better opinions but most of all, 35mm f/2 was half of the price of 28mm f/1.8 and 30mm f/1.4. This was the money I was willing to risk (and I am very happy with the results of this gamble).