Laser is more accurate, but it is focused. So it depends what you want.
For a backup sensor, a laser system just wont work because if it is angled incorrectly, you wont be reading the other car, you will be reading the ground under it, or even just miss the car alltogether.
Ultrasonic is a sound wave like sonar. So it is not focused like light, but it will hit anything in its path and give you a general idea of the stuff behind you.
So really it comes down to what you want to do with the sensors.
We will also be carrying a laser distance ranger, and it will have a maximum distance of 1.5m. So not really for backing up, but can be if you are careful!
What is it that you are wanting to do with the distance sensors?