I was in this predicament 10 years ago and bought the 100-400mm. I was testing against the tamron 150-600 as it is slightly sharper and the aperture is better through the range it was F5.6 further into the zoom that the sigma. Also at the time I was testing on DSLRs. Unlike mirrorless the F6.3 maximum aperture is an issue for fast action its outside of Canons F5.6 cross type range and when photographing birds in flight the tamron was significantly lower on hit rate than the canon but drop it to F5.6 and it wasn't far away. Main thing for me was the canon is built better, more compact, takes the tele and although 2/3s of a stop slower its still sharper than the 600 at F8. What I ended up doing was putting the 100-400 on the 7DII and it was an epic combo and kept my 5DIII at the time for wider. I traveled the world at this time frame for a year, traveling Africa for 3 months, Indonesia, SE Asia etc and it was a great combo. 160-640 is a great focal length and really anything over 600 can be difficult with atmospheric conditions and came up against this a lot in Africa with heat haze. The one downside to the 100-400mm is that its not really 400mm it is quite a bit shorter depending on distance to subject so it is a bit short changed when 400mm isnt a great deal anyway. If you can get close it is incredible and the such close focusing ability basically gave me a macro ability without taking the 100mm L Macro. Now we have such better AF with mirrorless the 5.6 limitation isnt a thing. I still own it and havent moved to mirrorless yet but with the R5II im very tempted and also tempted by the 100-500 and 200-800! Zoos aren't really my thing when you've seen it in real life I can't help feel sad seeing animals in captivity but understand it's an easy way to test. Main thing for me in the UK is birds and most of them are tiny so reach is really important and the 100-400 certainly is frustrating in this regard and getting close casually without doing a lot of prep and work is difficult so the more range the better! Also considering you often need F8-11 to get a bird in focus back to front and I often shoot at F8 or more these newer lenses seem intriguing!

I was in the same predicament and chose the Canon EF 100-400 ii for better sharpness, fast focus, great AF, great colors, canon product on canon camera and minimum focus distance (good for macro-esque shots). I have not regretted this purchase and still use it successfully on mirrorless cameras. Highly recommended 😊

I owe this Sigma (C). Im my humble opinion it is very sharp all to the end (600mm). Of course bearing in mind you take all precautions - tripod, fast time etc... I got some cracking pictures with it. I must say i never tried canon 100-400 hence I don't have comparison.

i have the sigma and a canon 400mm prime, and the canon wins hands down.

Currently have both of these and am deciding on which one to keep, so really cool to see your thoughts on it as well! 😎

I ended up getting the Tamron 100-400 instead purely on price as I got it for £325 and I'm more than happy with my purchase. I might at some point in the future look to get either the sigma 150-600 or Tamron 150-600 purely for the extra reach.

I am getting the contemporary in a few months time my local independent camera shop has suggested this to me I thought the canons own branded is the one to go for the 100-400mm mark II at the beginning I nearly was going to go for Nikon I know there big lens on mirrored is the 200-500mm very expensive if I did go Nikon thow I would of merged it with the old sigma 150-600mm apo and pretend i have the 200-500 I was nearly going to go for the 500mm if I can't afford the 600mm but it's one I really want to get Last year it was my first ever time I saw the Sony 200-600mm and I was amazed I saw it plus I wish sigma would go white because i have learnt a fact it can deflect the heat from the sun I might look for a a white lens cover to make it look like a white lens i have heard the suns heat can swell up the wirering inside the lens if it's a black shell and possibly break but dont know know how true that is anyway I will be looking forward snapping away this autumn I shoot currently with an old sigma 135-400mm apo currently but do have to use tripod but I use it hand held and high shutter even that the images will be soft

im thinking in buy one of these two lents for bird photos... what are your preferences ??