@murrayjames,

Sorry for the slow reply. 先锋艺术 and 前卫艺术 both refer to avant-garde art. There is no difference in general. But 先锋艺术 is more about 原创性 - the kind of art that never existed before and 前卫艺术 is more about the way how those artists express themselves - art concept and emotions.

That's how I understand it - hope it makes sense~