Hello, I'm coming into this discussion because I come one in once in a while for a visit to treklens and treklens' forums and always enjoy the beautiful pictures everyone is posting. It's an inspiration to me and I learn a lot just from looking.
Shelbeesmom, I own a Sigma 170-500 and I'm currently selling it because of the luminosity (I don't like the colours)and it's also a little too soft for my taste at long range. I would follow the advices on this thread and get a 1.4 or wait and get the Canon... that's in my plan too, waiting for the money!
good luck, hope we hear from you when you make your decision.