Strike is more of an agressive "I want to decimate the battlefield" kinda player. Whereas defender is well... Defensive. If you are good at staying alive defender is great for strikes or such if you are in a sticky sitatuion and want to live or if you need to revive a teammate in a bad spot. In the later though warn them that once the shield is done they will most likely die because of the concentrated fire on that area. If you are doing matches such as clash or rumble I highly suggest striker. But if you are doing matches like salvage or conquest defender is great for you to cap those objectives. For defender though I would highly suggest you keep the inside shield bonus health and get used to meleeing enemies that are dumb enough to get close to you. Or get good with a fusion rifle and charge inside and pop out last second to shoot them (can be highly dangerous though because it gives a split second where you are exposed). Also be aware that titan smashes are able to do their animation, land in your shield, and kill you. Also bladedancers are quite pesky (as you will soon figure out) if yo use defender super.