I had a Sigma120-300 briefly when I had a camera from the dark side but that was before they made an OS version. I seem to recall then that it was generally rated higher than the Sigma prime and it certainly offers more flexibility. I was very happy with it but had no experience of anything else.
Sigma have always had a bit of a reputation for less than perfect consistancy where as with the Canon you know what you'll get - but then you get what you pay for and there is a £3.5K difference to consider.
If Sigma made a 120-300 in 4/3rds I would have it like a shot but in the mean time I am waiting for a spare £6k to float past for the Oly prime
