canon if you have the money
sigma to save on your wallet as a rule of thumb
but my 70-200 OS sigma is pretty good, on a cropped camera it's perfectly good for what I need
the real issue is not which is better
but how much better for the purposes for which you are going to use it.
if you have a 600D it's more than likely that, like me, you are a hobbiest and you don't need 400mm prime images to get on the cover of Time magazine.
get something that you can afford and that you will get the most enjoyment out of. typically this is about versatility and ease of use
AF on sigma can be pretty good if you get the higher range lenses (I've found it much poorer on the cheaper walkabout lenses).
The canon 100-400? well I'm not a huge fan of it.
I've tested a copy of it (in unchallenging conditions) on my 40D against my canon 70-300 IS and although it was a bit better with more reach it wasn't OMG how can I live without L glass. (in the rain this would be different as you'd need weather sealing for longterm pro use)
At the moment I have a 70-200 sigma and a 2x converter. I would say it's not as good as the 100-400L but for me ...it's fine for the job (happy snapping airshows mainly, will get into some wildlife soonish) (might spring for a 1.4x one too to maintain more image quality)
For wildlife, the 150-500 is cracking from the pictures I've seen taken with it
and the new long reaching f2.8...I'd love to have a go with that one too!
have a browse around FLICKR and search for groups of shots that have been taken with the lenses you're researching.
at xmas, my problem is i do not no weather to spend the extra on a canon lens or save some money on a sigma lens, the lens in question is the 100 - 400mm lens. both have the same features but are about £400 differnece in price, does any one know if the picture quality from the canon is worth the extra money:shrug: thanks for looking
