Lens dilemma - 16-35/f' or 28/f1.8

I have a bit of a dilemma which lens to get and i would like some input.

I got a D800 a month ago, but coming from a D2X i lack a bit of wide angle for the full frame.

I narrowed it down to the 16-35/f4 zoom and the 28/f1.8 prime.

Concerns are,

for the 16-35:

- it's only f4, i never had anything worse than f2.8

- image quality

for the 28:

- will my feet provide enough "zoom" to get the shot i need

- i could get the 50/f1.8 to, but to much changing lenses might result in a dirty sensor