Recently, I was shooting the moon with my Canon 70-300mm and thought my Sigma used to do a better job. No, I'm not posting my recent moon photo's here (they're fuzzy, at best), but thought maybe I should do a comparison of the two lenses and see which does better at what.
Obviously, the Canon, which has image stabilization, will do better when motion is a factor. That aside, I put my trusty 5Dmkii on a tripod in our basement, locked it into place and started shooting a cereal box - using only ambient lighting. This was done at 300mm for both shots taken - with the same camera settings for each too. When I downloaded the shots onto my computer, I ensured each shot had the same white balance setting, so we're really comparing apples and apples. That's the only processing I have done on any shot here.
Then, I read in an article the other day about the Canon 70-300mm lens being a bit soft at the 300mm length. This would explain why my moon shots came out soft! Then I got upset, thinking that although I had tried to buy a better lens than my original Sigma 70-300mm, maybe my Canon was a wasted purchase? So, I decided to shoot an old TV remote control - but at 200mm instead of 'full-out' 300.
Below are the results. Keep in mind, I have cropped all four of today's shots, trying to keep them the same. Also, all four of today's shots were done using manual focus while checking that in 'live view' at 10x. I thought I had the remote control 'nailed' the same way in both shots - but as you'll see, the focal point is actually shifted very slightly in them. However, you can see that both lenses perform about the same in both shots. The dust is about the same on both (hey, this IS the basement!).
In short, I think both lenses are good, though was disappointed with the Canon not being so clear at 300mm. I am hoping that maybe outdoor shots at higher aperture settings will give better results. It should still give decent/better results than the Sigma will with moving objects and critters.