The interesting thing about Shapeways is that theres a bunch (25) of materials to choose from! Obviously depending on your application (The stainless steel is a expensive but intriguing option) you can choose for yourself, but i tend to choose the PA 2200 material. This is because its the strongest materai available. very light and reasonably cheap.

It would help if we knew roughly what product you are trying to produce, then we can pick a material based on the application/budget etc.