I got my hands on the 70-200 2.8 I Canon for a few times. Haven't tried the Sigma nor Tamron yet.
While doing some research I figured that all Objectives do their job relatively well. The main argument for Canon is the build quality and the low noise from the Image Stabilizer which is pretty loud for Sigma and Tamron I was told.

Personally for me the 70-200 2.8 is great but it's not applicable in any situation. If you have the money and you know exactly what you can use it for you won't regret the purchase.
Be sure to test them out with your camera before you buy them though.

Isn't there a reason the Canon is a lot more money? There's gotta be - I have a feeling the quality is probably different, material wise. But maybe not. I use a Canon 70-200mm 4.0 L IS and I absolutely love it. Good to see you're after the 2.8 though, it will make a huge difference!

This post was edited by Chantal7 on Jan 23 2015 10:08am

Canon zooms > all

But their primes not so much anymore unless its a specific prime like the 24, 100 macro or 135.

Just remember that the 135 will create a lot of compression and will require a lot of shooting distance. I find the 100mm macro is the sweet spot and i shoot half of my fashion portraits with it, the other half with a 35 sigma art (favorite lens in the world)

Downside to the 100 macro though is if you miss focus you may as well go make a coffee while it spins back around to hit haha

Rent the 135 and see how you go

Sigma has chromatic aberration, tamron doesent but has noisy af and build quality isnt in par with canon, canon costs shit loads of more but doesent have much better iq than tamron(other than minor differences in bokeh, bit better micro contrast and even smaller difference in sharpness), but has silent af and great build quality, also weights a lot more. Anyways better to go with tamron than sigma, and if you want to pay like double the price and have slightly better lens, then go with canon. But keep in mind that you could get shit loads of more important stuff with that like 1k you need to pay extra for small stuff with canon.

This post was edited by Antichrist- on Jan 23 2015 06:34pm