They do different jobs
Framing this as a choice is how it gets sold rather than how it gets installed. Epoxy builds real film thickness in one pass, fills minor imperfections and bonds extremely hard to a ground slab, which is exactly what a base coat should do. Polyaspartic resists hot tires, chemicals and UV, which is exactly what a topcoat should do.
Cure time
Epoxy takes 12-24 hours to recoat and up to a week to fully cure. Polyaspartic recoats in one to two hours. That single difference is why a two-day epoxy-base floor and a one-day full polyaspartic floor both exist, and why the one-day version costs more.
Temperature
Epoxy effectively stops curing around 10°C. Polyaspartic keeps going to roughly 0°C and its catalyst ratio can be adjusted to compensate for cold. In an Okanagan winter that is the difference between working and waiting until April.
How to read a quote
A quote naming an epoxy base with a polyaspartic topcoat, and explaining why, is a specification. A quote saying "it is polyaspartic, which is better" without a reason is a sales line. And if your garage gets direct sun through an open door and the quote lists a clear epoxy topcoat, that floor will amber.
Want this answered for your slab?
Free site visit with a moisture reading, then a written fixed price. Across the BC Interior.