Nitrided Steel
Recommended for percussive playing or for playing melodies with clear and defined sound of each note without merging between each other.
Instruments made out of this metal have warm sound, ceramic-like tone and lower sustain than ember steel.
Nitrided steel is resistant to oxidation so properly maintained instruments won't rust (see care & maintenance guide). However if the instrument gets scratched or will be used in harsh environments like rain or beaches that contains salt in sand & water some rust might occur over time.
Ember steel
Recommended for meditative playing: yoga, sound baths, etc. As it has very long sustain, the sound is very deep and relaxing.
Also very good as a second instruments for handpan players to make chorus or chords for their melodies with nitrided handpans.
It is a harder material and more difficult to work than nitrided steel, so for that reason they're a little more pricey.
Ember steel is a specific alloy of stainless steel that has a warmer sound, more controlled overtones and more sustained volume for having beautiful sounding experience.
Ember Steel instruments are not going to rust even if scratched.