I wouldn't say one is easier or more difficult, but when you're inside a costume and a mask, you have to endure heat - and, often, difficulty seeing. The vision is not very good in a mask. And you have to cope with that, as well as trying to think about this character.
If you stood me in a costume next to a computer graphic of the same-looking character, I think there would be a difference. And many movie fans I've spoken to would rather see an actor in a costume than CG.
If you just did a horror tone throughout an entire movie you almost, as an audience, can get a little bit used to it. But if you're laughing one minute and, you know, somebody's doing something quite horrific the next minute, it's a little more shocking.