I think I'm biased towards metal because Njoy makes them look sooooooo luxurious with the packaging and everything. I prefer metal for durability and such (less worry of ever getting broken), but glass is gorgeous and looks more whimsical. That's why I love both.
I like stainless for the temperature aspects. Like glass, it's nice and cool at first but I also enjoy how it seems to get warmer than glass, feeling almost hotter than body temperature after using it a bit. Lovely.