Wow, this got a lot more interest and replies than I thought. Thanks for everyone who took the time to answer, a lot of great advice was offered.
I saw a bunch of recommendations to invest in the canon f2.8 ii IS. While I have no doubts it's a fantastic lens, I can't help but make the comparrision to when I was buying my camera body a few months back. I was looking at a 6D vs. a 5Dm3 and also got a lot of ''get the 5D, it's the best'' (excluding the 1DX) and many people saying I'll regret the AF system in the 6D. But I got the 6D (for 1400$ new), and I'm thrilled with it. It takes stunning pictures, the AF has 99% been flawless and the few times it hasn't, I've made adjustments and got on with it. I borrowed a friends 5D for a day and while it's a better camera, I can't help but feel I got 90% of the camera for half the price.
Point of that was to say I don't feel I need the best of the best, I'm more looking for excellent quality without resorting to robbing a bank. Therefore I feel I just can't justify the extra money the canon will cost me. If I'm going down the used or refurbished (something I did for my other lens and I'll gladly do again), the tamron offers me savings of at least 600$.
As for the other options mentioned, the canon f4 IS looks fantastic, my only concern is that if i decide to attach a 2X converter, I'd loose autofocus. The converter would be a last option, but it would be nice to have that reach if I so decide. I'm also concerned about people posting that the m1 version of the 2.8 IS has much worse IQ. Can Tamron and Sigma really not make a lens in the same ballpark as canon? Both the sigma and tamron are newish lens as well.
Again, thanks for all the great replies, keep them coming.
Lenses are assets; bodies are liabilities. Look at how each depreciates in value.
I had a 70-200mm f2.8 and it was great. Tried my friend's 70-200mm f2.8 IS and it was similar, but with IS. Both are very good and you would be happy with them I think. You would also still wish you had the 70-200mm f2.8 II IS.
We both ended up selling those lenses. They weren't exactly what we wanted. The 70-200mm f2.8 II IS is.
It's not the sharpest lens I've used (the Sigma 18-35mm f1.8 wins there), but it is a joy to use. You can look at test charts and user reviews all day and you'll see that the cheaper options are not that bad. However, the 70-200mm f2.8 II IS is Canon's flagship and everything about it is just made to be great. The packaging, the build quality, the look, the autofocus, the performance, etc. It's a totally no-compromise lens. The best in class.
Why go for second best, when best isn't that much more expensive (and the total cost of ownership, taking depreciation into account, is less)? Why do you have to justify it to yourself?
If it's a business expense get the cheapest you can get (or what will offer the most return on investment), if it's a toy get the best you can afford. Either way I think it's an easy choice. Wait for a refurb or for a sale. Mine was $1820 new or something. Worth every cent and more.