Epoxy is rigid, hard-wearing and chemically resistant. It is the correct choice for interior warehouses, workshops and laboratories where abrasion and chemical spillage dominate.
Polyurethane is elastic and UV stable. It accommodates movement and thermal cycling, which makes it the only sensible option for exposed decks, balconies and car parks.
Specify epoxy outdoors and it will chalk and yellow within a season. Specify standard PU in a chemical plant and it will soften under aggressive spillage.
Where both conditions apply — thermal shock plus chemical attack, such as food production — the answer is usually neither: a PU-cement screed handles both.