I have a few thoughts here...
Firstly, "landscape" does not necessarily equal "wide angle", especially not "ultra wide angle", so you need to decide whether it is specifically wide angle or actual landscape shooting you want. Some of the best landscape shots I've seen were taken using telephoto lenses - the secret of good landscape shots is not "get as much in as you can".
If you want wide angle, there is a huge difference in field of view between 14mm and 17mm, so you need to decide if you really want such an ultra wide angle as 14mm. The widest lens I have is a 15mm (on a different system - not DSLR), and it gets used very rarely - it's hard to use such a lens regularly without it looking "gimmicky". I do have a 17-40L, and I find myself using it mainly around 21mm - 21 is a focal length that really seems to work for me.
Regarding f/2.8 vs f/4, I'd say there are far bigger differences between these two lenses for that to be a deciding factor - having an extra stop is no use at all if it's the wrong focal length for you.
So I'd say what you need to do, rather than asking other people which is the better "landscape" lens, is imagine yourself in your own preferred shooting positions and think about what angle of view and perspective suits your personal vision best - and unless you have actually done a lot of ultra-wide shooting, I would really not expect a 14 to fit the bill for most people.