Well, I pulled the trigger on a 100-400 today.
I tried the Sigma 100-400 today in the store, and after a couple of hours and much agonizing, I ended up buying the Canon 100-400.
It wasn't easy, mostly because the Sigma is SO much cheaper. Comparing photos between this and the Canon, and then looking at the results on a PC, I couldn't tell which camera took which photo, even magnified to 200%.
I thought the Sigma photos were very sharp, and the lens felt really nice in my hands. Ultimately, however, I bought the Canon because of many non-top-line factors. The first two were the most important non-top-line considerations:
* The minimum focus distance on the Sigma is 6.3ft; on the Canon it's 3.5ft. That's a huge difference on subjects like hummingbirds. Also, it opens up a lot more opportunities to blur out the background (400mm @ 5.6 @ close range).
* AF compatibility with 1.4xTC
- The canon locking collar is really excellent. You can lock the zoom at any level, instead of just at minimum zoom. It's also a collar, instead of a switch like the Sigma.
- At least on the copies I was testing, the AF on the Canon felt quicker and more confident.
- Tripod mount is a thing. I want to handhold this lens, but the option, if not critical, is certainly a consideration. Plus, the 100-400 has a really nice tripod collar/mount, with the detachable foot.
- IS mode 3 - this is where IS doesn't kick in until the moment the sensor is going to capture the image. I have no idea how well this works in practice, but it sounds like a lovely idea.
- The Canon 100-400L really does feel awesomely engineered. It's the difference between premium and ultra-premium.
- The Canon comes with a really nice lens hood. with a little door that lets you slide open and adjust polarizers. Ok, I will probably use this.. never... but it's still cool.
- Resale: should I decide to sell it, the Canon will barely depreciate.
And of course, there were top-line features; the wider aperture being by far the most important. I tried the AF thing with the subject moving towards me, and I didn't have a problem with either, but I will take the review at its word that when it's a fast moving object (like a dog), this becomes an issue. Not a big deal for me.
At the end of the day, I don't think I would have gone wrong with either, but the 100-400L is definitely a more premium lens (with the accompanying price tag). I don't think I would have been unhappy with the Sigma, but I knew I would always wonder if I would have loved the Canon more.
On the subject of price, and not that it was relevant to my decision, I was kind of stunned at the price difference between Sony and Canon lenses. With a little store discount for being a regular, the Canon came out to about $1,950 USD (about $100 less than Amazon US price). The Sony was like, $2,500!