I have heard really good things about both of them, and I know a few of the people from the Sussex course went onto become professional psychologists afterwards (which is what many of the conversion folk are after). I also think that the MSc or conversion students tend to be quite focussed and mature, which makes sense as they are paying quite a bit for it.
Prospects afterwards will depend on the kind of work experience you have as well as how you do on the course. Not sure about St Andrews, but Brighton I know is pricey.