Help with Constant Aperture Standard Zooms?

[gearsNcogs]

(there is another post of mine in the MF Lens section of the forum. I wasn't sure where to put my post, so I decided to split it into two, and place one here, and one there: http://forum.mflenses.com/are-there-any-constant-aperture-mf-standard-zooms-t55072.html. Apologies in advance if this makes it more confusing or inconvenient)

I'm normally more of a prime lens sort-of-a-guy (I primarily shoot video as well), but I've been finding that the majority of my work has been coming from documentary/reality TV/news reel style shooting, which lends itself to constant-aperture zooms far more than it does fast primes. As such, I've been looking into investing in a constant aperture zoom for my 7D. I've read quite a bit about the Canon 24-70 f/2.8 (both versions), as well as the 24-105 f/4 (I've used it as well).

I have a few questions, going forward, and they are as follows:

1) Is an f/4 too slow of an aperture for documentary video? Have you found yourself needing that extra stop of light in many instances? While I love the idea of having a constant aperture f/2.8, the significantly higher price seems a bit prohibitive, unless I go with a third party lens like the Tamron 24-70 f/2.8, which is close to the same price as the Canon 24-105 f/4 (truth be told, there will be quite a bit of saving up before I can afford any of these lenses in the first place).

2) Speaking of 3rd Party alternatives, I've read about the Sigma 24-70 f/2.8 as well as the Tamron 24=70 f/2.8, and from what I've read, I would be more interested in the Tamron (feel free to present a case for the Sigma though! ). Are there any 3rd party competitors for the Canon 24-105 f/4 that are worth looking in to?

Thanks for any advice that you have.