Lenore, who is installing the granite slab shower? Start there and make your priority be hiring a VERY experienced tile professional - one you have vetted carefully to insure that their qualifications meet your design requirements and product selections. Stone slab installations are not for the B team, and especially in a wet area. Can it be done? Yes, depending upon the structure over which a solid surface stone is installed and the methods used. We install solid surface shower walls regularly, but from an empty cavity which we build to meet all specific design and installation specifications, including the pan. A solid stone slab cannot be supported by the pan (it is not meant to support a slab wall). A slab wall is pinned and tied so it is self supported, not supported by a pan. This is a very complex installation. A LOT to consider. As one example, the thickness of your slab? 2cm. 3cm? (effects the weight load on the space). And I could go on. After you have carefully vetted the qualified tile contractor, have this conversation with them. If you question or want to clarify their recommendations, check back or seek professional advice from other qualified sources. Please consult with a professional onsite. Good luck with your project.

Here's what Onyx pans are made of (from website): Here is a little information on our chemistry: 63% of our products is a hydrate crystal--Al2O3+3H2O. Alumina is also the main ingredient in Corian© and all "solid surface" products. The main ingredient in our product (alumina trihydrate) is in roofing materials and the plastics inside automobiles to slow flame spread and as a smoke retardant. It is in the plastic coating on the wiring in your home (in a much smaller percentage). Our other 37% is a special polyester resin we developed over the last 22 years (and continue to improve) to get the properties which allow us to say "Yes, Guaranteed Forever". We all try to get properties which make our products work best for our applications. Our application is the bathroom. We believe our resin serves this purpose well. Corian© uses acrylic resin, others use various blends of polyesters or modified acrylics. We prefer our resin for our products and it serves us well. We use cultured marble type molds to cast our products. The solid surface industry casts sheet goods and fabricates their products. The cultured marble industry uses calcium carbonate as their main ingredient. Our Gloss finish does have a gelcoat on the surface (about 15 mils) similar to cultured marble. Our Gelcoat is ISO-NPG. ISO-NPG gelcoat is a tight molecule--very resistant to water penetration. Our Matte finish does not have gelcoat on the surface. We use a different blend of resin for Matte. Our products are real hybrids. The resin in the matte finish--we substitute 30% of the normal ortho resin with ISO-NPG resin. This increases the ability of the product to resist moisture penetration. This is necessary without the ISO-NPG gelcoat coat which is on our gloss products. And here's the link to Onyx's standard sizes. Here is a link that might be useful: Onyx shower bases

Can anyone comment on how the materials/construction compare in the context of a shower pan?

Just regular all-purpose bathroom cleaner.