metal vs nylon vs carbon fiber pushrods all have their advantages and disadvantages. It also varies from plane to plane and installation to installation in the same plane type. Important part is to minimize drag and flex. Keep it simple and secure both ends well to the servo/control. You will develop your own preferences over time due to both convenience, recommendations, cost and simplicity. A basic trainer doesn't need top of the line stuff but a 2 meter pattern plane does.