Which qualification will give me the best job prospects?

That depends on what job expectations you have. If you want to be hired in a managerial capacity (as project manager, for example) you should hold a Master's degree or doctorate. If, however, you want to work in software development as part of a team, a Bachelor's degree should be sufficient. In fact, having a Master's degree might make you seem overqualified for a job like this. At the moment, job prospects for Computer Science graduates are generally very good, particularly if students are qualified in some specific field and are not afraid to speak confidently about this. Because of the demand for specialists, specializing will give you better prospects than saying that you are prepared to adapt to any requirements!