Wouldn't the Sigma 35mm be better than the Canon? In terms of 50's, I've used both the 50L and Sigma 50mm 1.4 extensively and bought the Sigma, I just find the lenses colour rendition much better, focus more accurate and it doesn't suffer from the heavy (granted easily correctable) vignetting of the Canon either.
As to whether to go 35mm or 50mm it's all personal preference, though I would get more use out of 35mm than 50mm - especially since you already own an 85mm which will sometimes overlap with a 50's uses - I think it's important to spread your primes out or else they start feeling redundant (e.g. I use 24, 50 and 135).
Alexiumz can buy BOTH Sigmas for $1350..that is about the same price as ONE of the Canons....