70-200mm lens...which one & why?

F/4 IS if you want lightweight versatility, walk around outdoors, super sharpness.

F/2.8 IS II if utmost IQ and performance is your goal.

I own the 2.8 II, and it's just brilliant. But it's also damn heavy, and more often than not I find myself leaving it at home unless I'm doing something specific with it. Can see myself acquiring the f/4 IS in the future, without hesitation.

"If I had to rank the image quality of the Canon 70-200s, I'd put the 70-200/2.8 IS Mark II and the 70-200/4 IS in pretty much of a tie for first place, at the apertures they share."

I would have to pretty much disagree with this statement. They are not that close in IQ, at least not a tie. CA is nearly double in the f4 version vs the f2.8. I sold my f4 to purchase the new f2.8 and I am extremely happy with it. I think it is the best lens I have ever owned. It along with the new 24-70mm f2.8 make a killer combo.

That is if you have to have the best and you have the coin. The price of admission is pretty steep for this pair.
If not get the Tamron. I have it, too.

One thing about the 2.8 II i love is the focus is crazy accurate and fast compared to my old sigma 70-200 2.8 os, well worth the extra $. Also had the f4 is and it was great outdoors. if you don't mind the weight the 2.8 II is very versatile and can be used for almost anything in the focal length.