Which wideangle zoom or prime would you buy for D800 landscape?

Landscapes only, ideally can accept screw in ND filters (not grads). I've looked at the 16-35 and 17-35. The loss in stop of aperture probably doesn't bother me as it'll be stopped down? Can anyone help me decide? I'm possibly going to sell my 135mm f/2 DC (mint) to buy it and an old 24mm 2.8 prime so i'll have a budget of around £800-900 maybe more if I sell something else or save more. I'd even consider just keeping the 24mm and adding a samyang 2.8. I want the 24mm focal length as it is a staple for me; but I really want to be able to go wider, at least to 20mm.