Heya,
Despite all the mumbo jumbo, they're essentially equals in terms of image quality. Where they differ is in stabilization, and the Sigma has the fuller richer stabilization feature set. Otherwise, you'll hear one person state their copy is sharper than another, etc, and it goes back and forth. They're both plenty sharp for what they are and are again for all real world purposes, equals for image quality. The Sigma coming down to $1k for the C version is a big deal because it directly competes with the Tamron at $1k. That said, you can get both on deals. The Sigma can be had for less than $1k on deal, and the Tamron generally can be had 2nd hand for even less (I've been seeing a lot of $750's). For a lot of us, there was no option, and Tamron was it for a solid year, if you wanted an affordable physical 600mm. If I were buying today, and the prices were equal, I'd get the Sigma. That said, I have the Tamron, because again, I didn't have that choice when it was the only 600mm on the market for a year. And really, I'm still very pleased with the Tamron and I use it for birding and wildlife, astro, etc, on APS-C and my copy seems to be plenty sharp for me even wide open and I never did the firmware update and my stabilization works fine (I can pan with VC on and it works, so I never sent mine in).
That said, I bird weekly here in Florida, even in the cold. I more and more often use a shorter, faster telephoto on my 7D than my 600mm. I take my 600mm, but it often doesn't get nearly as much use as my shorter primes do. I actually prefer a 300mm F4 (if I could get F2.8, I'd use that instead!). But, it depends on what you're doing. I bird in swamp wetlands and the ocean shore here in Florida, and I often get close to birds. So close that 600mm is too long. I use my 600mm primarily for birds that do not allow you to get close (raptors in general). But more and more, I'm using a 300mm instead of my 600mm. And I do that for the speed of aperture, and the wicked fast autofocus that can keep up with a bird at 20 feet flying by, where my 600mm simply can't do that.
If I were to sell my Tamron today, I would just stick with my 300mm (I actually use a 200mm F2.8L and a 1.4x TC to make my "300 F4") on my APS-C and just save my money. My end game goal wildlife lens is the 300 F2.8L. I'll get an old non-IS one, one day. For now, have to get the kid through the dentist and get through tax season. That and I want a new boat. Priorities.... sigh.
Zooms are awesome for versatility, and 600mm for $1k is awesome. But, I still favor a good prime, it simply focuses faster, is sharper wide open, and I will take a shorter faster prime over a zoom in most situations and just let high resolution take care of the cropping. Again, after having owned a 600mm Tamron since it was released, I actually more and more use a 300 F4 more than anything.
I started with a 250mm.
I then got a 600mm when it was released. Used it nearly exclusively for a year.
Then started using a 400 F5.6, because I wanted the speed of AF and size.
Then I started using 300 F4 because I wanted speed of aperture.
What changed? My habits. I started out walking around and trying to shoot birds standing in the open like an obvious threat.
Then I started geting more into it and started going where my subjects were, early, and hanging out, or creeping up in a kayak, etc. I got closer to them.
So then I realized I didn't need 600mm. I started kayaking with a 400mm and walking around and using blinds with a 400mm. I still got closer.
Now I'm using 300 F4 because I can get that close and I love the F4 aperture look on telephoto. I can get closer still, but 10~15 feet is probably enough!
At this point, I'm just going to wait and get a 300 F2.8 as my final lens. I thought I wanted the Sigma 120-300 F2.8 OS, but I also want to keep it as compact as I can, and while that lens is awesome, I know I don't need anything shorter than 300mm and I would basically get it for the 300 F2.8 end and it would be useful with TC's. But, it's a big lens, bigger than the prime. So I think for me, the 300 F2.8L is the prime for me in the future. I salivate at a 500 F4L or 400 F2.8L, but ultimately, they're big, heavy, long and having shot 600mm, I've learned I don't need that long of a lens here in Florida, and I'd rather have the hand-holding-friendly 300 F2.8L, and I can always put a TC on there for a 600mm F5.6 on a gimbal if I want.
But again it greatly depends on what your situation is. For planes, I think the Sigma 600 is the way to go, you want the best stabilization you can get, so either the 600 Sigma C or a Canon 100-400 II at double the price. For birding, that depends on the species and how you do it. If you just walk around and shoot things if you happen upon them, get the 600mm. If you study subjects, creep, camp, use blinds, etc, you can get a shorter faster lens.
Here's the kind of proximity and BIF action I can get (this was yesterday) with a 200mm on my 7D using a 1.4x TC. I keep focus just fine (I set sensitivity of servo higher instead of lower when doing this kind of proximity). So this is a 7D with a 280mm F4 lens on there (200 f2.8L + 1.4x TC), and even some song birds:
IMAGE LINK: https://flic.kr/p/DDZffX 
IMG_0461 by
Martin Wise, on Flickr
IMAGE LINK: https://flic.kr/p/D6uYbz IMG_0391 by
Martin Wise, on Flickr
IMAGE LINK: https://flic.kr/p/DBEQxL IMG_0472 by
Martin Wise, on Flickr
IMAGE LINK: https://flic.kr/p/DcSyxU IMG_0448 by
Martin Wise, on Flickr
Of course, I still love 600mm for doing things like this:
IMAGE LINK: https://flic.kr/p/DdAkDQ IMG_0353 by
Martin Wise, on Flickr
Choices are tough!
Very best,
