The longer barrel was developed to meet US laws. If you use the real P90 barrel (10" or so), it requires approval by the ATF and most states do not allow SBRs (Short Barreled Rifles). So in order to maximize compatibility, FN made them with 16.1" barrels. In Michigan, my PS90 is considered a pistol due to its overall length (26.3") due to some funky laws.
The PS90 receiver appears to be cast aluminum with a cheap paint. After loading/unloading the magazine a few times, the paint was coming off the receiver. It sounds like the DTP rail is a perfect match: both are cast alum and both are painted.
The P90 and PS90 share the same parts except:
barrel (length and flash suppressor)
trigger and hammer group (P90 is Auto / Single / Safe, PS90 is Single / Safe)
They both use the same receivers, except the PS90 is Olive Drab. It can be dyed black or duracoated black though.