I would expect that the breathabiltiy of the wood would be an asset, and the non-porous nature of the plastic to be an issue.
Or, it could not matter at all. Without a proper study it's all just blah blah blah.
Plastic is beter than no shoe tree. Though, if you spend more than $200 on a pair of shoes it shouldn't be an issue to spend an extra $10 or $20 for some crappy wood shoe trees (at very least).
I don't expect there would be a disadvangtage to plastic when it...