I would say you should look at what work would be available to you with the benefit of the course (though you would probably have to do more).
Computer security would I'm sure produce some very interesting work that would be continually changing.
Computer forensics, always make me think of those who are rummaging through computers for deleted c-h-i-l-d p-o-r-n for the evidence to be used in court and that isn't a prospect that I could see me doing. Though, I guess there is also the fraud side of computer forensics.
Whatever you choose you have to look at what kind of work it is likely to and would you be happy in that field of computing.
Sorry not to go for one (though both would be an option as suggested) as without any idea of the course syllabus it is hard to say.
