Abstract, these are both great lenses IMO. I have the 100-400mm and couldn't be more pleased. I contimplated getting the Sigma 50-500mm (Bigma) as well mainly because of the reach and versatility. However I went with the Canon for the following reasons.....
-Got a great deal used, manufactured in 2004 and used less than a year.
-IS (image stabilization) Though I'm learning to use a tripod more I hate carrying it around. Most of my shots are hand held because I like to hike with my XT and the 100-400mm in my low pro mini Trekker. It just fits inside , but it's very snug. When not extended ti's fairly compact.
-Versatility, the Sigma has more range but the Canon's 100-400mm range is good enough as this lens I use mostly at long end from 200-400mm. Great for the zoo and wild life.
-Image quality is awesome, from colour and contrast and one of the sharpest Zoom lenses out there.
-Pushpull zoom: I was hesitant at first but I like it, I find it easier to zoom in and out and there is a zoom lock mechanism that hold it in place.
I could go on about this lens but those are the main points. I've never used a bigma, well once in the store and it was a little heavier but just as good in quality, but no IS which was one of the main factors when deciding to get the Canon.
If you can afford a new one by all means go for it, however there are a few used ones floating around. If you are on a limited budget, the Bigma would be an excellent lens and if you don't mind carrying around a tripod.
I know there is some amazing bird photogs here I :hail: to your knowledge, many thanks :hug::
is the imaging show worth going to for an amateur or have you got to be in the business to do it?
