The front end is much more difficult. Why? Because that's what the users see - it is the application as far as the users are concerned. Difficult in this context isn't referring to technical difficulty, it's the difficulty of having to deal with the end users and all the problems that entails. From a technical perspective the front end is also moving faster, so there's more churn and more things to keep up with. Finally, there's less research. Front end development hasn't benefited from decades of academic research. It's a mess.
The back end? That's classic computer science. Decades of research, many classes of problems solved, a much slower evolving landscape. That doesn't mean the back end is easy, but at least it's well-known and can be mastered.
Which should you do? That's entirely up to you. Some people enjoy the messiness of the front end and the constant technology churn. Some people prefer the stability of the technology stack on the back end and not having to deal so much with end users.
The back end? That's classic computer science. Decades of research, many classes of problems solved, a much slower evolving landscape. That doesn't mean the back end is easy, but at least it's well-known and can be mastered.
Which should you do? That's entirely up to you. Some people enjoy the messiness of the front end and the constant technology churn. Some people prefer the stability of the technology stack on the back end and not having to deal so much with end users.