The first review I Googled said this...
http://www.pbase.com/lightrules/30v35_2nd
"Conclusion: The Sigma is slightly sharper (center of frame) from f1.4 - f2.8, controls flare and CA better, is significantly smaller and lighter, and costs less than 1/3 the price of the L. The Canon has slightly better extreme border sharpness from f1.4 - f2.8, controls barrel distortion and light fall-off better, is a tad quicker with AF, and has a closer MFD (and higher magnification). In terms of build and build-feel, they are comparable (solid metal mounts, no weathersealing for either, both can take some bumps and bruises, but drop either from 6 feet up onto concrete and both will need a service trip, etc.). When you consider ALL the strengths and weaknesses of both lenses, then factor in the price on top of everything, IMO the Sigma remains the one to have for APS-C standard primes. It's very sharp from the start (f1.4), has excellent color reproduction, is small and lightweight, and costs under $400 USD. It's for this reason that the 30 EX gets my recommendation over the L for APS-C photographers."
TBH that surprised me as I'd expect the much more expensive Canon L to shade it, but I don't know how old this Canon lens is and as some of their lenses are a bit long in the tooth now maybe that would account for Canon's less than clear, if any, victory.
I used to have a Siggy 30mm f1.4 and I was always amazed by the negative reviews and always suspected that most were down to user error (there's a lot of that about I'm sure...) and badge snobbery. That the reviewer found the Siggy sharper in the centre doesn't surprise me as mine was basically sharp from the off. I like centre sharpness and compact size but if buying today I'd consider FF compatibility too and obviously that brings the Canon to the fore.
PS. Just Googled the Canon 35mm f1.4 and got a date of introduction of 1998. If that's true it's now a 13 year old design and maybe things have moved on.