ABS is the stuff used to make OEM bumper, not PU... LOL.. I wouldn't buy a car using OEM PU made bumper.. must be a cheap sh1t car to use PU as OEM bumper..

for "beautification" purpose like sideskirts and "aero parts" either one should be fine, not as main load/impact bearing structure..

how can PU be stronger than FRP? LOL... there is a difference between "strength" and "flexibility".. both are different engineering qualities..

more details here... cheers.

http://en.wikipedia.org/wiki/Acrylonitrile_butadiene_styrene

http://en.wikipedia.org/wiki/Polyurethane

yeah, my mazda '8 kit uses ABS plastic, the advantages are that is dimensionally consistent per molded part while flexible enuf to withstand impact/scrapings over speedbumps/side kerbs (of coz the paint will gone case... ) ..
but the high pressure injection mould costs are high so mass production quantities are required to amortize the mould cost..

while FRP has very low mould costs but dimensions are not as accurate, depends alot on the guy preping the surface for final paint/finish...
and also, is more rigid hence, may "crack" under stress or being flexed. I guess it's lighter weight than plastics like ABS...

PU, that;s poor man's ABS replacement, more flexible than FRP (but not as strong as ABS) but the outcome will depend on on the mould's workmanship.. maybe decent looks or may be crap appearance...similar to FRP also, installer dependent quality..

juz my 2 cents cheers..