The pads work well, but do not have the amount of adsorption capacity that the granules have simply because they are impregnated on the sponge material. Using the granular material in a reactor is much more superior in terms of surface area and capacity.

I have used both, but I prefer the granular for a larger tank. If you need to remove phosphate in a small tank (<25 gallons), either do water changes or use the pad. If you have a larger tank, then use the granules, I have found that the sponge will trap detritus that will fall off when removing the sponge from the tank.

Al vs Fe is debatable. Ferric may aid in plant growth while aluminum will not. Both are industry standards. Fe is gaining more use for the reasons that AZDesertRat has stated.