Material selection of heating element
The heating takes place throughout the entire volume of the resistive element (ignoring effects at the connection points) so if you use a low thermal conductivity material, you'll end up with a temperature gradient between the center and the edges, where heat is presumably being convected or conducted away.  Many high power resistors use resistance elements that are thin in relation to their length - either a ribbon or a wire - so there's just not a lot of length for a big gradient to develop.  All other properties being equal, I would choose the higher thermal conductivity material as a smaller temperature gradient would reduce differences in electrical conductivity (temperature coefficient of resistivity), internal strain (from thermal expansion), etc between the core and the surface of the heating element.  But there are other properties to consider like corrosion rates at elevated temperatures, formability, and compatibility with other materials such as solders or protective coatings.