The courses sound a bit different, so find out as much as you can about what modules you'd do and how many options you'd have. There might also be differences in the number of contact hours, the balance between lectures and other teaching methods such as seminars and the balance between exams and coursework, so that's something else you could look into. In terms of the uni, it depends what kind of atmosphere you're looking for. Royal Holloway is in a tiny village called Egham where there's hardly anything to do, I've heard the student union isn't all that and it's really expensive, whereas although I've never been to Cardiff and don't know much about it, it's bound to be more lively being a city and it's probably cheaper. I don't know if it's a campus uni though, but Royal Holloway is.