I've owned the Sigma 70-200 2.8 IS, the Canon 70-200 2.8 IS I and finally the Canon 70-200 2.8 II.
About my copies, from de "worst" to the best :
Ma Sigma was actually better than the first Canon version. And the 70-200 II is the best one overall. But not that much. It ain't night and day. It will never ever change your pictures getting version I or II....differences are not that huge...
Is is more about personal satisfaction about owning the "best ever".
The Canon II is better as i spend less time on my computer, sharpening and contrasting a bit some part of the pictures. Pictures are great straight out of the body. I insist, i actually must not anymore sharpen them. But, once you post-processed them, differences between these lenses are not that impressing.
Moreover, I regret the Sigma because of the black outfit and cause zoom ring and focus ring are inverted as on the Canons. The zoom one is on the front of the lens. Wich, i find, is really more confortable and stable.
I also consider that Sigma stabilization is faster than the Canon one...
If you wanna be proud of your "white lens", get a Canon.
If you want the "best of the best", get the Canon II.
If you want a good 70-200 2.8 IS but you're a bit cheap, get the Sigma, i'll perfectly make the job.
But don't get an f/4 or an f/2.8 non-IS. If you're a bit short, sell your 50 1.4 or wait. But that would be sad to get an f/4 or f/2.8 nonIS if you know that you'll sell it in some months to upgrade. You'll spend a lot of time looking for the best deal, you'll spend time selling your lens and you'll spend time -again- looking for the best deal on the 2.8 IS.
Since i've my Canon II, i don't shoot better pictures and IQ don't look "that" better, no one except me -and photographers friends- could tell the difference.
However, once you have the 2.8 IS II, you'll never be able anymore to accuse the gear when you miss a shot, haha.
70-300 is so dark....f/5.6...seriously.....? I often find the 2.8 zoom lenses to be so dark when you use to shoot with f/1.2-1.4-1.8 primes...i don't even imagine having to bother with an f/5.6 one. Moreover, fixed aperture are definitely easier to shoot..
The "ego" factor is important, you know it. Self-confidence is a key for good shots. Seduction/charming is the key for great-portraits. These elements are linked. So, if a part of your self-confidence needs the white lens, get it. I suppose that it always sucks for you ego to meet some guest who boasts about having better gear than you when shooting a wedding.
On the other hand, your IQ needs 2.8 and IS to get the best shots too.
To conclude, i wouldn't even consider an f/4 or non-IS lens. Sigma must be considered if you're a bit short on the budget. Tamron might be too but i d'ont know it.
That's only my personal opinion. I give it here, now you do whatever you want.
Wish you good shots.