Depends on your play style. You like getting a pile of kills and generating a lot of orbs for team mates??? Striker is for you. Get close, make a mess. If you prefer a more supporting role, protecting your team, providing much needed buffs, and producing an overshield that will mitigate some damage you take for a period of time, then go defender. I recommend striker for missions where protecting your team isn't a top priority (Strike playlist, story missions, weekly heroic) Defender is better in Raids, and nightfalls. This is where defender really shines. Protecting teammates from devastating attacks, providing weapons of light for killing atheon, and Ward of Dawn is an excellent enemy distractor in Crota's End.