What camera do you use and which Filter holder system are you going to get?
If you have a crop sensor maybe the 85mm would be OK, they are half the price of the 100mm, but you may get vignetting in the corners. If you are full frame or want to go wider than 16mm in the future then I would go for the 100mm.
I use 100mm ND Grad Hitech Filters with a
Lee Filter Holder system and a
105mm Heliopan Circular Polariser. I got them when I had a 1.6 crop 40D Canon with 10-22 lens, to avoid vignetting in the corners. They now work fine for my full frame 5dII and 17-40.
Sorry about any erroneous links(in green) the forum software puts in its own and I can't be bothered to keep on correcting them