In my view a BMus keeps more options open and if you still want to make a living playing your music then you still can do after graduating.
That's probably slightly confusing advice in the modern era, as Conservatoires confer BMus degrees - so for example, Birming Conservatoire AND Birmingham University (totally different institutions) both award BMus degrees, the conservatoire for their performance-based degree and the university for its more study-of-music based degree.
Conservatoire study is absolutely based around performance - individual lessons and groups will be the core part of the teaching, though there will be lectures on history, teaching etc, the core of the degree is about playing your instrument. Larger groups - orchestras, big bands etc - will be led by faculty members, and the final year will be geared towards 'showcasing' to potential employers / opportunities.
The course may also be focused on a very specific type of music - classical, opera, jazz, rock.
Music degrees are about the study of music - its history, composition, theory, different composers etc etc. There may be a performance element but much may be essentially extracurricular and not part of the course assessment or core teaching.
One option for someone undecided might be the Manchester Joint Course - a bit of both! www.alc.manchester.ac.uk/music/study/courses/joint-course-with-rncm/ Quite a small course, and very competitive - also predominantly aimed at classical musicians.