Problems I've read about for both:
Lifeproof
-Charging door latches break causing the phone to lose the waterproof feature as the door no longer closes. (Remedy: keep getting replacements when this happens.)
-Sound issues - people hear you clearly and the sound has a tin can effect.
Defender
-Built in screen protector doesn't sit flush on the screen and has a noticeable gap causing the user to press harder in the screen. Also reports of the screen protector leaving marks around the home button. (Remedy: most have removed the built in screen protector and use their own glass protector.)