for me, PU always, since ive aredi xperience with fibre bumper 2 times. frendly speaking, fibre will easily crack at the screw hole where tie the bumper to the car.. 2-4 month crack aredi, but since ive use PU, aredi 1 year without any crack or "tertanggal".

fist to recognize fibre, feel the inside part of the bumper/skirt, if its rough surface, confirm fibre, coz inside surface of PU or original plastic is smooth, second, try to bend a bit at some of the area like the end of the bumper/skirt, where u can squeeze it gently, then when u squeezed it and "creek" sound heard, that is confirm a fibre, as for PU or plastic, there no "creek" sound.

just my 2 cents