Yeah, Tamron is a pretty good brand. The difference is just glass quality, and construction. You want the highest quality of glass, because lower quality glass will decrease your picture quality. The better glass will do that too, but not so much. (shooting through layers of glass and air, so you want the clearest you can get).
Just try to stay away from anything that looks too good to be true. Quantaray isn't that great, but then again, I use a Quantaray UV filter because its cheap. On something like a polarizer though, you really don't wanna skimp, so go with something a little nicer.
It's almost impossible to make perfect polarizing foil and perfect circular polarizing foil. It's expensive to laminate the polarizing foil between two pieces of glass in an optically perfect way. The short cuts are in the use of one piece of glass, not using perfectly parallel glass, not properly bonding the glass to the polarizing foil, and in the use of cheap polarizing foil.
Circular polarizing foil needs to do the following:
Eliminate linearly polarized light of the 'wrong' direction equally across the spectrum;
Pass linearly polarized light of the correct direction equally across the spectrum; and
Circularly polarize the linearly polarized light coming out of the linear polarizer completely at all relevant wavelengths. This is the hardest to accomplish, but it is the least important.
All those three are difficult to accomplish, and imperfections may lead to colour differences, albeit slight ones that may not matter in many circumstances.
Note: a circular polarizer is a linear polarizer followed by a 'delay plate' that turns the linearly polarized light into circularly polarized light (which behaves like 'unpolarized' light in many ways)
I also have a dirt cheap Tiffen ($20) that I love because it makes some really wild and distorted colors... and I like wild and distorted colors! If you are trying to accurately record a scene, it is horrible... but if you want to make something that looks like it is just bizarre, it is great.
This is not an HDR, it is with a D40, kit lens, set to vivid with the cheapo Tiffen.
I went with the slim version of the B+W circular polarizer MRC Kaeserman blah, blah, blah. Figure your lens is only as good as the cheapest piece of glass in it. Nothing against them, but i am not throwing a $20 filter on a $1000+ lens. The glass in crazy thin. Hopefully to diminish any further light loss or distortion. Works great though, albeit a bit hard to adjust due to the slimnest.