A servo-saver is designed to absorb some of the impact on a collision that would normally damage the servo (ie hit the front wheel on something that would cause it to turn rapidly and strip the servo)

Are you sure your car doesn't already have a servo saver?

Failsafe brings your car back to a pre-set condition (usually neutral or brake on) in the event you lose radio contact, or the battery drops below a certain voltage (if your batteries get knocked off the car, it won't help, so you better get a throttle return spring). It basically prevents a runaway. These come highly recommended (mostly by folks who have had to chase down runaways)