At the risk of annoying those who do have a preference for a specific type of first stage design, the simple answer for most divers is NO the design type of the first stage does not matter. Piston type first stages are simpler designs with a lower parts count and thus generally considered to be more reliable. Environmentally sealed diaphragm type regulators are generally considered better suited to diving in near freezing temperatures. Keep in mind that much of the opinion often heard expressed is a result of theoretical considerations or manufacturer marketing, not based on the actual characteristics of modern scuba regulators. Today, both piston and diaphragm first stages are very reliable and high performance balanced versions of both designs can flow more gas than the diver can breathe.