I have the Lexus OEM Parking Sensors and am very happy with them. The only advantage of the Quanam system is that is has a visual readout but from what I have seen its pretty ugly (nothing like the one in mercs so you probably wouldnt use it).
The Lexus ones have no visual indicator at all but it is not really necessary.
Also, the quanam sensors appear to fix onto the bumper, whereas the Lexus sensors are embedded into the bumper.
With regards, to painting them, i'm not sure, but mine are blue to match the car. I'm not sure whether Lexus painted them but they look more like they just came blue.
All in all, i think the lexus sensors leave a more professional and 'meant to be there' finish.
Also, there are 4 sensors in the Lexus system.