I recently upgraded to a 6D and I am loving it. I purchased the Tamron 24-70 f/2.8 and just received it. I am looking for a zoom lens as well and keep going back and forth between these two lenses and further, considering an extender as well.
The prices are pretty close to each other. So what would folks recommend?
It really depends what your needs are.
The f/4 IS is a lot lighter and more compact and nicer for general run-around. It's also a trace sharper in the center and more noticeably sharper (on FF, on APS-C it's not much different) at outer parts of the frame. It has IS which can useful in many cases with static or semi-static subjects.
OTOH f/2.8 gives just a bit better subject isolation and stops moving subjects better. If you want it mostly for sports I'd go for the 2.8.
If you plan to extend to 280mm a lot, the 70-300L does very well and would make better sense than the 70-200 f/4 IS think. I made the swap myself. The f/4 IS is amazing, but so is the 70-300L.
I originally had the 70-300 IS non-L as a light run-around lens and a 70-200 2.8 non-IS as a top quality lens and a sports lens (especially for indoor sports or night sports, but it was a bit better even for daytime sports although not by a whole lot in that case but better, at least on the aps-c cam I was using then). I stopped shooting indoor sports and sold both for some cash back plus a 70-200 f/4 IS. I made do with it as a wide field sports lens. My 300 2.8 was my main field sports lens anyway. I eventually gave the 70-300L try and wow it really was good. At the edges it did about the same on FF. In the center and both set to the widest aperture the 70-300L was capable of it actually beat the f/4 IS at 70mm, noticeably for sharpenss (although it definitely has worse lateral CA there) and just slightly beat it at 200mm. It lost at 135mm though. It was the same at 100mm and just slightly worse at 165mm. It did noticeably better than the f/4 IS +1.4x TC, less CA, sharper, more bite, more contrast and 50% faster AF in that case (both lenses bare the AF speed is basically the same).
All three are super good other than the 2.8 non-IS going a bit soft at the edges if used on FF anywhere at all near wide open.
At 200mm f/2.8 the 2.8 IS II is the best, amazing, then the 2.8 non-IS very good (other than edges on FF) and the 2.8 IS easily the worst at center frame (although not worse than the 2.8 non-IS at the edges) it had a lot less micro-contrast bite and lot more PF at 200mm f/2.8 than my 2.8 non-IS did.
at 200mm f/5 center frame I'd put them 70-200 f/2.8 II > 70-300L > 70-200 f/4 IS > 70-200 2.8 non-IS > 70-200 2.8 IS. It's not that the 2.8 IS is bad at 200mm f/5 center frame though, other than some PF, it's VERY good, the others are each just a little tiny bit better than the one below and all really beyond exceptional. The top three there are all so great at 200mm f/5 across the frame that IQ really doesn't matter, you get what you need in terms of IS, aperture, size, weight and can afford.