The tray is going onto a concrete/screed flloor and the surrounding height can be adjusted to suit. Which is the best thickness of tray to go for? Basically, why are there differing thicknesses of tray?
The only real benefit from thicker tray is the slope. Thicker tray is steeper, so if you're going to have a lot of water going through the shower area it can be beneficial to have thicker shower tray. However even 20 mm thick tray will provide sufficient slope so no matter where the water drops it will still be drained.
20 mm is the most popular thickness and recommended especially for timber floors (because floor boards are usually of similar thickness) and by proper installation you can simply replace them. If you have concrete floor and more space thicker tray can be a good option as well.
I am happy to help if you have any questions.
David 020 7193 0953