Terraining is more of an art form, whereas GUI is a form of scripting. They are pretty different, so it is tough to say what is harder than the other. Basic terraining is very easy to learn, but advanced techniques take artistic perspective, which isn't straightforward to learn. GUI, on the other hand, is straightforward. You can learn what specific functions do, and how to perform particular tasks. But there are many concepts, so it'll be something you'll constantly work on.
So I don't have a clear answer for which is harder. Terraining is simple but takes a lot of practice. GUI is straightforward, but has a large amount of content. I suppose it depends on what you enjoy most.