Are you ready?
I got the Canon 70-200mm L-series IS 4.0 lens. NOT the 2.8.
Here is why.
First, money was not the issue. I had $1800 cash to spend on a lens, but wanted to be absolutely sure that the lens I bought was the one I really wanted, regardless of cost.
It came down to weight vs. sharpness vs. speed. I had to ask myself, how much is speed really a consideration? I decided that speed wasn’t the primary consideration, and that sharpness and weight were the top two issues. BUT, having said that, the 4.0 buys you two whole stops with the image stabilization enabled. (On still shots anyway). This narrowed the competition a bit on speed.
The 4.0 is also *considerably* lighter than the 2.8, which is important when one is traveling and covering lots of ground with their gear, like I did for my Sonoma/Napa shoots. Trust me, the lighter your camera bag when trying to stuff it in a tiny overhead compartment on a tiny plane, the happier you will be.
Back when I was asking for advice I recalled people saying “you get what you pay for” and “the 2.8 is a much better lens”.
Which leads me to sharpness. For those who say “the 2.8 is a much better lens” the 4.0 is actually much sharper. If you equate sharpness with quality then which lens do you think is better?
The 4.0 is so sharp that the images that come from the lens *need no sharpening* post-process. Simon touched on the fact that some of my images from my Sonoma shoots are overly sharp- this is totally right as I am discovering that the images coming from the lens, again, need no extra sharpening in PS.
Need proof? This is what was the deciding factor for me. Scroll your mouse over the little arrow above the middle of the image to compare lenses:
the digital picture 100% crop comparison between the 4.0 IS L and the 2.8 IS L
The 4.0 may not be the best lens for everyone, but I can tell you it’s perfect for my needs. Also, after doing my research online, I learned there is small but significant movement of people who are selling their 2.8 lenses for the 4.0. Interesting.
Now, having said that, I somewhat regret buying the lens in the first place. Why? Because it’s on the opposite end of the spectrum from where I am happiest shooting- the wide end. There is something about a wide dog photograph that I find immensely appealing, and the lens that I have been really wanting- my dream lens if you will, is the 16-35mm 2.8 L-series. I have a feeling that if I had bought that instead it would be on my camera at least 50% of the time. I love, love, love my 20mm prime lens, and only wish it were an L-series as it’s pretty soft. But, I have been using it more and more recently, and am reminded every time I look at the images why I love it so much.
BUT, I needed a telephoto at least 10% of the time, so I am glad I got it. But I can promise you, the next lens I buy will be the 16-35. I just need to come up with $1500 first, lol.
Thank you to all of those who helped me with this, especially Craig, for generously sharing image samples and unbiased advice. And to Mia, for agreeing with light weight vs. fast speed.
Lens stuff can be so complicated, but when you get ones you really love it just makes shooting that much more fun. 
