One thing to keep in mind, is that the render engine doesn't really care how many polygons it is, to a point. Everything is bucket and pixel based at that point, and the calculations involved to render it are more important than the mesh density.

A low poly mesh with diffuse, bump, displacement, and a normals map could actually take longer to render than a mesh of higher density and a single texture map.

Like others have said, you have to consider all of that.

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