Currently I have the 70-200 f4 IS and I must say it's a very good lens. When I started the big topic back then (2.8 mk2 or f4 IS) I decided to go with the f4 IS due to smaller size and price. Now I'm craving faster apertures at longer focal lengths and once in a while wished that I just got the 2.8 back then. The f4 feels a bit slow now and just doesn't give me the background separation I want or more light when I need it.
I'm starting to enjoy primes more now so I was considering picking up a 85 1.8 and a 135L to compliment my f4 IS. I'm renting the 135L now and enjoying the images I get with it, but it does feel a bit limiting at times so I was going to use the 85 for those times. But if I go this route, I know the 7-2 f4 will rarely get used and I don't want that to happen.
This is when I thought of this option: for almost the exact same price of adding a 85 and 135L, I can sell my 7-2 and buy a 2.8 mk2. I've seen the work from that lens and it seems to be very close to what you can achieve with both 85 and 135. And many times I see people selling the 135L after getting this lens. This option may be more versatile but maybe it won't because of the size and weight?
Add 85 1.8 and 135L
I've owned both and there ISN'T a remarkable difference in the two lenses. Is the Mk II better? Yes, but marginally so. Depends on how picky you are, I would guess, but if you only have a few lenses and are looking to expand, that's what I'd recommend! 

