Purely for landscape get the f4 but if you will use this for anything thats moving in low light, grab the mk2.
IMHO as im in the same kind of boat as you, i would go for the mk2 purely as its more versatile and even those the f4 is good enough for landscape, well so is the f2.8 + more.
Make sure that if you do get the f4, dont look back and wonder if you should have got the f2.8 instead!
i say, if you have the money, then why not?
alternatively you can grab the f4 IS and a 135L combined for similar price.
yes i probably have thrown more choices to you making it harder LOL but please consider "all" possible options i.e f4 + prime? or the mk2 alone