As a 16 year-old you must wear a belt in the back seat; when you turn 17 the law doesn't require it except in the front seat. However, you would be a total fool not to wear it. Just in the last 10 years I have known at least 3 people who survived horrific accidents because they were belted in. Two were in a single rollover accident in which the car skidded on its roof more than 100 yards, shattering the windshield and peppering both of them with bits of asphalt through the hole where the windshield was. When I was young I worked with a man who had his knee completely rebuilt after a collision when he was in the back seat of a Mustang that hit a bridge abutment. He was thrown through the windshield (after taking out the shifter with his knee) and hit the abutment with his head.
One of my friends worked as a paramedic in Utah for 10 years. He told me that in all that time, much of it responding to injuries on the highway, he only saw two fatalities who were buckled in. Both had struck the back of flatbed trucks so the truck bed came through the windshield. On the other hand, his first call was to a single car accident where the passenger was ejected and died at the scene from massive head trauma. Her boyfriend, the driver, was crying uncontrollably.
The choice doesn't seem that hard to me.
www.dmv.org