Graduated Neutral Density Filter - Ring or Rectangle

I am interested in a a GND filter. I have seen a few that you can directly mount onto the lens (ring mount) without an adapter. You can also purchase the kind that fit onto an adapter (Sing Ray, Cokin).

I would like to purchase the ring-type filter.

Any recommendations?

You can not adjust the ring filter up or down to accomodate or adjust to a skyline.

Get a square one !

With a circular filter you can only ever have the dividing line in the same place (generally the middle I suppose). If you want more or less covered by the ND area, you're stuffed.

Square ones are much more adjustable and you can place the ND where you want it.

Rectangular, as stated above. You want to adjust the grad to the picture, not vice versa.