Back end pays better, but front end is pretty close. It depends on too many factors honestly. If you're good you will make more than if you're not, so get good and don't worry about what pays more, just do what you like more.
PS it also depends on location, USA pays better than pretty much anywhere else on average for tech work, especially if you're in FAANG or a pre-ipo startup (think AirBNB, Snowflake, ect).