Plastic. Glass breaks, although I've broken a plastic one too. It isn't critical to the brewing process, only to taking a measurement, it doesn't need to be perfectly sanitized, just maybe at the top where my turkey baster may touch it while taking a sample (also plastic).
The only glass I use in my brewing are the carboy and the Ehrlenmeyer flask for starters. Note that both are where fermentation takes place and need to be clean and sanitized. Glass is best for that IMO.