This has been discussed many times before. Wide open F4L IS is a hair sharper than the F2.8L MK II. No lens is perfect, it really depends on what your needs and usage patterns are like. As far as I am concerned, weight is important, 70-200 F4L IS + 24-70 F2.8 MK II weigh the same are 70-200 F2.8L MK II. Now to some that's not an issue, to me it is so I opted for the lighter and just as sharp F4L IS. Sometimes I think of upgrading but the weight/size factor is a deterrent.
Then again, if one's a professional shooting wedding and/or events, needs that F2.8 then the weight becomes secondary.