Eraser, definitely. I use [mechanical] pencils all the time, and make mistakes in my writing all the time as well, so erasers are something I've pretty much got to have around. In addition to that, I don't find white out particularly easy to use, it still leaves a noticeable (in my opinion, since I like everything as close to pristine and perfect as possible), spot on the paper, especially if the paper is lined, and the smell isn't particularly appealing to me either. =/