The difficulty or ease of a trail is purely subjective. Some get bored with the straight and unexciting trail the road offers. This may make the trail seem harder. Whilst others like the adventure presented by a rocky trail. Some see a rocky trail, with its full body workout as an enormous task they are not willing to try.
There is a consensus that trail running is significantly harder than road running. The thing about trail running is that it gives you a lot of different things to look at.
Distractions along the way can motivate you and they can also help the distance go by faster. Running on a plain road or city street can get pretty boring and make the distance seem longer. Running on a trail requires a lot more concentration and watching where you step than road running.
There is a definitive answer as to which trail is easier to run on. It is a subjective matter depending on what you value. Both trail running and road running offer benefits that are unique and can not be compared, but rather be chosen based on personal preference.