When looking at kitchen cabinetwork doors you should be looking at more than just a sample, but how the edge is detailed. The edging for polyurethane doors is seamless as it applied like paint and the "paint" just wraps around the top and side edges. You can't wrap a melamine finish to the front and four edges of a door, so somewhere there will be a tiny seam between the front and an edge.

Some melamine doors have rounded side edges (which are wrapped) but a square detail to the top and bottom edges where there will be a tiny seam between the front face and top edge. With your choice of Jamaican Walnut this seam should be almost invisible, but is easier to spot on lighter finishes such as Laminex Parchment.

The Jamaican Walnut in matt should be harder wearing than the polyurethane. Some polyurethane colours yellow slightly with age and exposure to sunlight and are almost impossible to match if you ever need to replace a door. But you could also have the same issue if the manufacturer of Jamaican Walnut stops making it. For resale the polyurethane is considered more prestigious if it is still in good condition.

Why don't you ask your builder to see a sample of a polyurethane door and a melamine door so you can see the edge details?