The problem with aftermarket pads is that it is very unlikely that they will have been tested on the vehicle you're fitting them to.
OE pads will have been specified as the result of cooperation between the vehicle manufacturer, the brake manufacturer & the friction material manufacturer to ensure that they perform acceptably in all areas of performance - friction level, pedal feel, fade resistance, noise, etc. Even then, sometimes they get it wrong - see all the threads on MiTo brake squeal!
Squeal is a phenomenon which is still not totally understood in the industry - a material which is virtually silent in one application may well squeal like a pig in another. Disc material does matter - pads & discs should be considered together as a "friction couple". While discs are made from low-tech cast iron, certain trace elements in the iron can affect performance. There are several different options for reducing or eliminating squeal - rubber or steel shims, rubber paint, chamfering & so on, but these are really only palliatives.
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