I think you mean the Canon 28 1.8 - Canon doesn't have a 30mm prime lens. Does he say why he thinks it's better? All I can find on his site is where he says it's better but there's no reasoning or photos to back it up.
When I bought my lens I debated a lot between the Canon and the Sigma, mostly because I wasn't sure if I wanted to go with the 3rd party lens or not. But everything I read, especially from people who had used both, the Sigma won out pretty much hands down. It's sharper and has more contrast than the Canon. I had heard that some people have focus issues with 3rd party lenses (the Tamron mentioned above has those issues too) but I didn't have any issues and even if you did, a simple calibration from Canon will fix the problem.
Here's a review that recommends the Sigma over the Canon complete with side-by-side comparisons:
http://www.pbase.com/lightrules/28v30. Scroll down and click on the Main Tests: Resolution thumbnail - it will bring up a series of different tests with side-by-side results.
Ultimately it was these two threads that sold me on the Sigma. I was WAY more impressed by the Sigma photos than the Canon photos. I can't say I've regretted my decision at all.
Sigma 30 1.4 lens sample archive
Canon 28 1.8 lens sample archive