Of course the production of 1 kg of wood causes less emissions than the production of 1 kg of plastic. In some products you would need about ten times as much wood as plastic. Plastic can be recycled, wood cannot. This is not always justified. If a product consumes energy in the use phase, there is a ten to one chance that energy consumption is dominant .
People normally underestimate the environmental impacts of energy.
A coffee machine uses 300 kWh electricity during its lifetime, equal to 60 kg of oil. During production only 2 kg of plastic is used. A designer can influence the product life time in several ways. More important, try to design the product in such a way people will feel attached to it. Many products are not thrown away because they are broken, but because people got bored with them.