I'd go with aluminum i have no problems with mine. I put a paper gasket on the top of the insulator and one at the bottom to insure it is sealed good. No leaks and problems with heat at all for 3 or more years.
The original one (made out of some hard weird material) was junk! It never sealed any good and broke in 5 pieces when i accidently dropped it on the floor. :mad:
