stillen is a 20% underdrive for every accesory as it is just a crank pulley. NST is a 10% underdrive for a/c, and power steering, but comes with a 10% overdrive pulley to get the alternator back to normal speeds. Stillen also offers a factory size pulley but i don't think that is what you are asking about.
In order of performance: stillen 20% > NST 10% > NST 10% + alternator pulley
reverse that for the order of driveability
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