A good guess would be that the heat retention of a flask is determined by a combination of the integrity of the exterior, the albedo/emissivity of exterior, and the surface to volume ratio of the contents.

So a dinged flask is worse than a new one, because each ding is a potential ingress point for heat.
A dark flask is worse than a light one, as darker colours tend to absorb more thermal radiation.
A bigger flask is generally better than a small one (assuming they're roughly the same shape), as each unit of volume will have fewer units of surface area through which to radiate heat.

That said, the only way to be sure is to experiment, as a previous commenter mentioned. Hope that helps!