Some info on materials can be found here:
http://www.engineeringtoolbox.com/li...ents-d_95.html
http://www.engineeringtoolbox.com/th...ity-d_429.html
Material - thermal coefficient [(10-6 m/(m K))] thermal conductivity [W/(m K)]
Aluminum 22.2 205
Glass, Pyrex 4.0 1.05
So.. in principle glass is 5x better than aluminium in terms of expansion coefficient, but aluminium is 200x better in terms of conductivity - so less internal stresses, due to uniform thermal gradients inside material.
I imagine glass is much easier to figure and polish compared to aluminium... or not?
I think the homogenity of glass is definitelly better controlled.. and there is no issues with chemical stability of glass at melting temperatures.. while aluminium may even burn in normally oxigenated atmosphere.