Originally posted by Newtophotos In order to take better photos, is it better to purchase a polarizing or ND filter for both of these lenses?
The short answer is NO. Polarizers and ND filters have specific functions, use them when you need a particular effect. But they will not magically make your pictures 'better'.
Originally posted by Newtophotos Should the polarizing lens be circular or square?
I prefer circular if you mean round screw in rather than square like a Cokin. There are also circular and linear polarizers as noted above but that is different.
Originally posted by Newtophotos Should these two filters ever be stacked?
'Should' has nothing to do with it. If the effect you want requires them to be stacked then you stack them, if not then you don't.
A CPL reduces glare and reflections from non-metallic objects, like water. It only works well when you are pointing it at 90 degrees to the sun. So pointing it directly into the sun really has little affect.
A ND filter reduces the amount of light that goes through the lens, this is used to get really slow shutter speeds for things like dreamy misty waterfalls.
One other factor is that adding a filter (or two) to the front of the lens reduces the image quality to some degree. So only use a filter when it will do something that you need. And one of the big downsides of filters is that in some situations, like shooting into the sun, they produce ghost reflections in the image. So in general I avoid using filters when the sun or any point light source is in the frame.
And another thing is that on very wide lenses, (wider than say 20mm on APS-C) a CPL can cause weird looking skies with dark areas and light areas that looks unnatural.
My recommendation would be to make sure you have and use the hoods for your lenses and if you want get a CPL for the 18-135 but I wouldn't bother for the 10-20. Just MHO.