In most turntables like Teres, Redpoint, Galibier, the materials chosen are the preferences of the designers/owners of the company. These choices are typically made by comparative listening to the TT, using the different things, and choosing what they like the sound of. All these TTs are good, and will vary a bit in sound, due to the variations that each company uses. They have all emerged from one starting point, The Teres Project, so they all have similar roots, and probably have some sonic similarities left, even after materials/design changes.
Regarding the materials chosen and the effects on the sound, it is difficult to determine exactly how a given material will sound, unless you also consider it in conjunction with the rest of the overall design. While I like brass as a material, it is not sufficient to just apply brass "willy nilly" at any location on the TT and expect better results. There must be a reason to use a material in the application and position for it to work out right.
For example, brass, when used in the proper formulation and shapes, can have excellent energy transfer properties. But, you have to have some place for the energy to come from, and someplace for the energy to go, for an energy transfer media to be of use. So, the TT must be engineered with these vibration transfer pathways in mind, or else the material will not function as desired. In the case of hardwood, this material is very good at dispersing energy thoughout its diversely-grained internal structure, and will also tend to transfer some of that energy along the longitudinal grain pattern. This application needs to be carefully considered when using wood in a TT.
Plastics work in a variety of ways, depending upon their formulations.
So, it's necessary to know what your materials can do, and how to apply them to the best effect in the application. There is no way to just say that "brass is better", or "wood is better", unless you can understand how it is being used for its purpose. In some cases, brass may be much worse than another material, if it is not used correctly. Generally, a knowledgeable person can tell by just looking at the way the pieces of a product are constructed and assembled, as to which materials would be productive in the application. In many cases, the product could benefit(or get worse) by changing the shapes and assembly techniques of certain parts, when they use a different material.
It depends on what the designer is trying to achieve with his design. The better he knows what he's doing, the better the result will be. He will use the proper materials and the proper geometries, and the proper assembly methods for his design to be correctly realized.
