膠重定木重

It depends on which type of wood and which type of plastic you are comparing.

The density (mass per unit volume) of wood ranges from 0.17 g/cm3 (Balsa) to 1.37 g/cm3 (Lignum Vitae).

For plastic, it ranges from 0.9 g/cm3 (polypropylene) to 1.39 g/cm3 (polyethylene terephthalate)/

But in general, you could say that plastic is heavier than wood.

You may refer to the information in the following web-pages for wood and plastic:

http://www.simetric.co.uk/si_wood.htm

http://answers.yahoo.com/question/index?qid=20080916141035AAcgYlN

梗係膠重過木啦！
膠係實既！而木可以浮水，是纖維組織，梗係輕過膠啦！
不過如果係發泡膠就一定係木重過膠羅！

plastic because the mass of the plastic is greater than the wood