Jump pack gives you fly, so you can move faster and jump out of close combat and still shoot.
Cataphrachii makes you harder to kill. It's tougher armor.
Welcome to the forums! This is more a tactic question than a rule question,
YMDC is for when a rule is confusing and/or not able to be easily figured out. Please try to be sure and post your questions in the right place, more people will see it then.