#1
Which material makes a string resonate the longest? Phosphor Bronze, 80/20 Bronze, or Nickel?
#2
they will all be about the same. the duration of the sound depends on your guitar more than anything. saddle material, nut material, bridge pin material, and bracing pattern. those factors are the more important when it comes to sustain. also, thicker strings(mediums) tend to give more sustain than lighter strings(light). other than that, theres not really too much you can do about sustain.