I looked at both a couple of years ago, the Sigma got very good reviews and was recommended to me. Then Canon launched the F2.8 mkii (with a lovely price!!!).
I thought about it, discussed it and decided the Canon was the lens I really wanted, my worry was I'd buy the Sigma and then end up selling it as I wanted to get the Canon version ...
My advice would be to think about what you ideally want and then save up for it and if you can, try them out and see which you prefer...
I did this with the UWA lens and for me, opted for the Sigma 10-20mm over the Canon version - for what I wanted and having tried them both, I liked the Sigma and opted for that.