The only way you will know if they really work is to test your water and mark down the results. ( Try to do the test at the same times and not too close to feeding time or not right after a water change. ) That way, you will see with your own eyes whether the level is rising or falling.
As for which pad is best, I've only used the Poly Filter pad. It has never failed me so I had no need to try anything else.
I had another cory fish die. It must be my water quality. The high Nitrates in the tank of over 80 ppm Nitrate. These cory fish were supposed to be hardy, so I have read on it. I made a big mistake in buying more fish. I hope they do not die. Also, I will look into the poly filter on Amazon, of they have them, that is.
I just look and Amazon does have the poly filter for Nitrates and Nitrites.