Box style capacitors are available in both metalized polypropylene and metalized polyester. The box style provides better heat transfer to the ambient environment. If you have an application where heat dissipation is a problem, then a box style capacitor of a given dielectric might be the better choice. However, there are many factors that determine the temperature rise of a capacitor, ESR being one of the most important. For high reliability, choose the capacitor that gives you the lowest temperature rise. It is quite possible that a high quality cylinder type polyester capacitor could provide a lower temperature rise than a low quality box style metalized polypropylene. If your application does not subject the capacitor to high current, then the box style probably provides no inherent reliability advantage.v