I'm not really familiar with compostable plates, but unless they eat from large plant leaves, the plates are probably made somehow. And to make them, someone probably uses resources, like energy and raw material. And they have to be shipped to Etsy.

Compare this with porcelain plates which can be re-used and last a long time, but have to be washed, which uses water and some energy.

Taking all of this into account, I cannot say which is more environment friendly.