Neither.
In the first book, it shows the wizards of UU in a freezing winter. Ponder Stibbons proposes the idea of the thaumic reactor which kinda goes bad. But HEX saves the day by creating the Roundworld Project...
Or us.
It looks at the science of our world by using the wizards, so it is neither one or the other. They're interesting reads.
TSOD covers the creation of the universe, the solar system and so forth. TSOD 2 covers the evolution of the 'mind' from 'mindless' creatures, while TSOD 3 covers evolution.