Bro...you are trying to compare apples to orange. You can eat both but they taste difference and you eat them in none similar ways. Same goes for the ND and Polariser Filters.
Reachme2003 basically hit it on the nail regarding both filters.
Let me try to elaborate on it further since I am a hardcore believer of Cir Polariser and nothing to do in the bloody office and a boss who refuse to give half day today! hehe...
Neutral Density Filters ( ND)
Usually you can buy them in a set of 3(or more) or sold individually. They come 3 to a set, for example, because they come in varying degree of dark tint. They would usually use EV as a means to indicate the strength of the tint on the filter. Neutral means that in terms of colours, this filter will not have any impact on colours of your subject or scenery like shifting of colours, contrast..etc. What colour you see is what colour you will still get just like you never use a ND filter. What this filter was made for is to (trick as I call it) shift your shutter speed range thus alterating the exposure period. It can be anything from 3EV to 8EV for example.
Example I: I am shooting a single stalk of flower against a background of leaves. You want to blur the background out so you want to shoot at your widest apecture..say f1.8. So you set your camera to apeture priority mode, select f1.8. Unfortunately it's a very bright day and it seen that at f1.8, it is too bright and out of your shutter speed range in relation to your f-stop. The only way to get it to within the cameras shutter speed range, you need to close down your f-stop maybe from f1.8 to f8. But doing so will increase the DOF which would instead of creating a blur background as you wanted, you now have one that's in sharp focus. Making your shot too busy or take the attention away from the flower. In this case, we can use a ND filter say with a 6EV factor. With this filter over your lens, exposure latitude has changed and thus your camera is now 'tricked' into thinking it is alot darker so now the speed it calculates would be lower by 6EV that should put it to within your shutter speed range but still allow you to keep to your F1.8 apeture setting for that shallow DOF.
Example II: Using a ND filter to let you shoot in slow speed to create special effect shots. The best example would be those waterfall shots where the water seem to flow in a white and blurry flow manner unlike shots where the waterfall flow is frozen in time and every splashing drop of water is caught in mid air very clearly. Again with the ND filter you trick your camera into capturing the same scene at a much slower shutter speed creating a waterfall shot with water that flow smooth white and blurry fashion. You set your lens to the smallest apeture f22 and the speed the camera suggest is 1/30 as it is a bright sunny day. So by putting an ND filter of 6EV for example. Now your camera could be suggesting that the correct shutter speed for the shot is now 1 second or maybe longer.
Thus ND is very much like "sunglasses" for your camera.
Polarisers: To some extend, you COULD use the polariser as a ND filter but barely because it is really too lightly tinted to be of much use(giving you maybe about 2EV). BUt yes it is possible. That about all the similarity this filter will have with the ND filter.
What polariser filters does is amazing. ( And not surprisely, they are the most expensive among all kinds of filters) Till today, I always try to use one when I am shooting all my outdoor scene or any shots with too many reflective surfaces. Most folks I notice when talking about this filter, always refer to how much more bluer or darker shade of blue the sky gets when using this filter. Yes it does that of course....and more! The filter works by cutting out reflected glare by filtering the lightwave passing through the filter. It does not work automatically. You need to turn the rim of the filter which will rotate the polariser to locate an OPTIMAL refraction angle to cut off the maximum amount of glares from the sky, water, objects..etc. Of course while you rotate it, you have to be looking into your viewfinder to detect the changes in colours, refelction on surfaces..etc For example you can cut off the glare from the water of a swimming pool so much that you can almost not see water plus maybe clearer view of everyone's swimming custome inside the pool now
Use that on leaves or any surfaces (even non shiny surfaces) and you can reduce or cut reflection off the leaves or surface too and thus improve coloration. When I shoot some of my product shots, I sacrifice abit of light exposure by using the polariser on my lens. Of course, I could spend more time adjusting the light(s) but sometime when you lack the time or just abit lazy...it does allow you to save time and get away with it. So what they say about polariser is true about the colour. Actually that is half right. They dont change the colour to darken it. They just take away all other distracting glare and light reflection from the colour surface and what you get are the true colours on the object or scene recorded on your film or CCDs
That in a nutshell is why I love the polariser. It can also help to get rid of glare or oiliness from a model's skin...not 100% but again..it helps.
Still need more info about polariser...I just found this site..not too bad. And got some pictures too heh.. wish I could put some of my up but I am stuck in the stupid office and can't get home early!
Typical chinese boss.....
Okay that's my two cent for 2004...