I think plastic heats up more; and it tastes terrible after getting hot. It's been 104 here in s Ark. and my hummers are not having any problems with glass. I do have some plastic feeders.

Most of my feeders are plastic but I do have one large glass feeder. When you are talking about 100 degrees there is really no way for keeping it cool other than changing the mix more often. I dont think hummers really care if the mix is warm compared to cool but I feel they do care more about freshness which also refers to changing the feeder more often. My glass feeder sits in shade untill about 2:30pm in the afternoon. Also if the mix starts to look different or not clear I will change it.