Materials isn't my strong suit, so someone correct me if I'm wrong, but I believe it's impossible for a porcelain or ceramic brewer to retain heat better than a plastic brewer. Ceramics will always have a higher thermal conductivity than plastic and will thus always be conducting heat away from your slurry to the surrounding air. This is a very general rule, as certain ceramics could have a higher air content or could just be engineered better, but fundamentally the materials will have different properties.
That said, an obsession with heat retention isn't necessarily going to result in a better cup. If you prefer more body or maltier notes in your coffee a porcelain brewer will help with that since your slurry will spend longer at slightly lower temperatures. If you prefer brightness, acidity, and sweetness then you're probably going to want to go with plastic. But a ceramic brewer with a lid, or with the proper brewing technique, can easily produce a much brighter, sweeter cup than say, a Clever dripper with a single pour and no lid. Also remember that at a consumer level your grinder is going to have by far the greatest effect on the result in your cup regardless of the brewer. Upgrading my grinder made my Chemex cups dramatically sweeter, despite its horrific heat retention properties.
Edit: Just to caveat, temperature obviously has a huge role in brewing coffee, but to be clear heat retention is not the end all be all. The following is purely conjecture, but I think Scott Rao's recent experiments with the DE1 helps prove somewhat the theory that pour over extraction is largely the result of the initial moments of ground immersion. Focusing on the temperature of the water coming out of your kettle will offer orders of magnitude more returns on the end product of your cup than focusing on the temperature of your slurry.