Depends if you're talking from the perspective of the hardware/OS developer, or the application software developer. For the system, AR is definitely harder. For the software developer, AR is "easier", if you're like most developers and are only treating an AR headset as a bad VR headset and just plunking 3D content down in some random place on the user's floor. You don't have to create the environment--the user supplies that. And you user's input modalities are so limited that you don't really have a lot of options on what sort of interactions you can design. For smartphone AR apps, you're basically just making a standard smartphone app that happens to have a 3D view in it. For headset AR apps, you're basically just making the "hunt-and-peck" version of what you could make on modern VR systems with much higher fidelity control systems.