Polarizing filters and NDG Filters

Ok I'm a little confused. I did some research but can't come up with a solution. I was under the impression that NDG filters helps compensate the loss of sky or ground detail depending which area your exposing. But apparently a circular polarizer does the same thing? I'm confused at to what the difference between these two filters are and their purposes, and if I need both. (I only want a filter to help maintain strong sky detail). Filters for flare/uv/etc I could care less about.

A Circular Polariser or CPL as they are commonly referred to will also help to eliminate (or reduce) reflections on things like water, windows, shiny surfaces as well as reducing exposure by a few stops, i get mine in a week so i will be able to give a better answer but have a read of these pages which have helped me quite abit.

edit:

oops links
http://www.cs.mtu.edu …ide/filter/pola​rizer.html
http://www.tiffen.com/​polarizer_pics.htm
http://dpfwiw.com/pola​rizer.htm

They actually serve two different puroposes; the effect may in some areas overlap. A grad will allow you to darken the sky only, by 1 or more stops (depending on the grad you buy), but it'll do this in any direction. It'll do nothing about glare or reflections.

A polarizer will control glare and reflections, and when aimed at 90 deg. to the light source can darken the sky. However it doesn't darken the sky uniformly; the effect is strongest at 90 deg. to the light source, and fades to practically none as you move away from that. It'll also darken the ground in front of you, which a grad won't.

So if I were to only buy one and my concern was to keep as much sky detail as possible, I'd go with a CPL or an NDG? If its NDG let's say a 0.6.

Depends. If I were only getting one, I'd get a polarizer as it's more generally flexible. If I were only getting one exclusively for keeping sky detail, I'd maybe go for the Grad. But you also need to consider what your horizon's like. If it's busy, you'll want a soft transition grad; if it's fairly distinct, get a hard grad.

Thanks Jon. So would something like this be what I'm looking for?

Hoya 46mm (Moose) Warm Circular Polarizer Glass Filter

http://www.adorama.com​/HY49CPLW.html

Aside from 46 mm won't fit any of your lenses unless it would mount on theSD700 and you don't need warming filters with digital since you can just dial in a slightly higher Kelvin temperature than you'd normally use (about 200-250 Kelvin more, or around 5800 k for daylight) to get the same thing. I'd suggest you forget the "warming" part and look for a good multicoated filter (Hoya should be at least SHMC; B+W MRC or Heliopan).

Jon thanks for all your help so far. I came across these two filters decently priced. What do you think?

Tiffen 77mm Circular Polarizer USA made w/Deluxe padded pouch

Tiffen 77mm ND.6

Tiffen? I wouldn't if I could manage a good Hoya (SHMC), B+W (MRC) or Heliopan.