I've used both and the Nikon is slightly sharper, however (this is just my opinion) both those lens are too wide for landscapes. All you get is lots of small things far away, I very rarely go wider than 18-20mm and if its necessary take a couple or 3 shots and paste them in PP.
As has been said you don't need the faster lens as you'll be shooting at f8 - f11 or more.
Cheers
Andy