The 20mm f/2.8 looks ok for the price. Could be the right balance for wide FOV (on the 5D Mark II) for landscapes, and fast enough for nightscapes.
There's some sample images with the combo of 5D Mark II and 20mm f/2.8 here.
I might still get the 17-40mm L for landscapes too. One day
The 16-35 is like $1400 at DWI - a good price and I'd love to get that lens but I just can't justify spending that much all at once. The 5D M2 is more important to get now