I don't own a Sigma, but I would definately get this flash over the 420ex if I had it all to do over again. The 420EX is just doesn't cut it on the Drebel.
I plan to one day upgrade to the Cano 20D so I sprung for the Canon 580EX which I hopefully will get for 399. This is $200 bucks more than the Sigma but has the ability to have an external power pack for $140 which is much cheaper than the Quauntum power packs. This combo give you 400+ flashes and recycles form a full discharge in 1.7 seconds.