Glass Reinforced Plastic is much stronger than XPS which is eXtruded PolyStyrene, while XPS is cheaper to purchase in most cases GRP shower trays work out less expensive to install as they are self supporting and don’t require extra materials for installation.
GRP Shower trays like Tough-X can have a finish of Tiles, Mosaics, Alto Style Flooring which covered pretty much every installation type. XPS can take tiles only due to load pointing, if the tile is to small the pressure can be transferred to the Wetroom Shower Tray and work lose.
If you are installing your wetroom shower tray upstairs the vast majority of UK builds will be timber floor and timber joists. Tough-X GRP are the best solution for this type of wetroom installation, Due to it be self supporting and not requiring any underboarding between installation times and materials are reduced.