Well, I've just been on the hunt for a 2nd hand Canon 70-200L f2.8 IS lens. Couldn't find anyone wanting to give one up.
So I went and bought a brand new Sigma 2.8. It's not IS -cause I can't wait that long
However, one thing I noticed in all of these threads is people saying that the f4 is not as shallow as the 2.8. That maybe so, but you can still set your 2.8 @ f4
By purchasing the 2.8 you are giving yourself more options. And adding a IS into the mix, gives you even more.
And the weight is really not that much. The sigma I own is 1.5 kg and I can hand hold it for a quite a while - I rented a canon one which is the same weight and ran up and down a football ground sideline with it for the whole match. I left my monopod too near the sydney football team and they chucked it under their water table for the match! So I had no choice - and I was fine.
And I don't even have big muscles!
I think if you can, go for the 2.8 IS and if you can hold out for a couple of months - go for the Sigma - there is not much difference at all from the Canon - especially on the crop sensors.