If you're connected to a Wi-Fi connection and use Apple Music, the music you listen to will be streamed at 256kbps. This is considered to be "high quality" according to Apple.
When connected to a cellular network, however, the bitrate will fluctuate based on connection quality. It's safe to assume that the cellular bitrate, while never officially established, is always less than the 256kbps that you get over Wi-Fi.
This is basically confirmed by the appearance of a new settings switch found in iOS 9 beta 3, which allows users to force high quality (256kbps) bitrate streams over cellular connections.
But what if you're running iOS 8.4, which lacks the option to force high quality over cellular?