I would guess clay would be better than plastic, because of the evaporative cooling. Miltassias are hard to keep alive in warm weather. Every little bit probably helps.
__________________ May the bridges I've burned light my way.
I've been growing mine in a plastic pot in a relatively cool place (the coolest I have in summer gets 28 ºC/82 F). It has been growing really well, so well that after the last repot, two years ago, it needs another one also due to its size.
I'll try to find a good clay pot.