To be honest, the only time I'd buy a third party flash is for off camera flash work. I'm sure Sigma has come a long way with TTL metering and errthang, but just buy a used Canon so you know it will meter and such.
Agreed, for the price I would also stick to Canon for on camera. If you want to go off cam at some point you can pick up cheap manual flashes from a number of companies.
I hope your philosophy about the auto setting on your camera is different... I have never owned a ttl flash. Manual all the way. You can get so much more creative.