I'm an installer for the Parking Dynamics sensors.
There is a great deal of argument regarding which type of sensors are best;
Ultrasonic (visible type with heads)
Elecromagnetic (invisible type with strip fitted behind the bumper)
The quick answer is neither is best but here are some pointers to influence your buying decision.
Ultrasonic sensors require the bumper to be drilled.
If there is rain on the sensor heads they may not work properly
If the car is any other colour than black, the sensors will need to be colour matched to the car. If that isn't done with the correct primer/paint at the correct temperature the paint will chip off after the first few jet-washes and look rank.
Be very careful what make sensors you get, Steelmate and Laserline are good, there are 101 chinese makes that aren't.
I do still fit steelmates to vans with black bumpers, but on cars now I fit the Parking Dynamics sensors.
Electromagnetic sensors require that the bumper be removed to be fitted.
If you try and do it without taking off the bumper and cleaning it really thoroughly then the strip will peel off after a bit and then be intermittent. I clean the inside of the bumper shell with alcohol spray and then use a special sealing tape and plastic sealant spray after fitting the sensor strip.
To get them to work properly lots of things need to be taken into account, the height of the sensor, the thickness of the bumper the position of the crash-bar. If you have never fitted them before, you're likely to get one of these factors wrong and the sensors will give false warnings.
The buzzer on the parking dynamics kit is a bit too quiet to be fitted in the rear of the car and need to be fitted in the front to be effective.
So in conclusion
Your best option is to get the Parking Dynamics sensors fitted to your car by a professional
Its not the cheapest option, but if you were cheap you'd be driving a Kia
If you're in the Northwest, I'd be delighted to offer my services.
Reverse Parking Sensors