My Missus has the electromagnetic strip type on her car.
As you say, very sensitive. Make sure the feed wire to the strip is well cable tied to stop it moving as this acts as a detector too. The slightest wobble as you go backwards will cause it to bleep.
Relys on movement of the car. It has normal bleep..........bleep sound when activated, then bleeps get more frantic as object is approached. If you stop 4" from obstacle, normal bleep.........bleep will resume. Don't attach the strip past the start of the curve of the bumper.
Ultrasonics are 'active', in that if you're close, the bleeps will maintain fast rate, whether or not you're moving.
I fitted £25, four sensor, buzzer type from Ebay. Fiddly getting wires through foam bumber filling, but well worth it.
Apply masking tape in rough area of sensor, MEASURE AT LEAST SEVEN TIMES and use small drill to start. Then use supplied hole saw. It's nerve racking!
When testing, don't do what I did and accidently whack the bumper on a gate post!