I've seen no evidence that points to triangles giving a better UV result. I do know that quads are typically far easier to unwrap, because you can select edge loops, and create seams much faster. Edge loops usually cannot be selected easily or reliably on triangle faces, as you know. I'm not sure it matters as far as what algorithm is being used. The algorithm of the unwrap does not depend on how the faces are divided as much as it does how and where the seams are created.

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