Own both. In a vacuum, Afm edges out MB due to greater depth and more challenging play.
Big 6 is the MB's siren song, and you mostly will find yourself dashed against the rocks. I find
myself either playing for that or playing for MOR, MB and MoshPit. Love the MoshPit intro animation.
AFM has more interaction between modes and goals. Love the international and especially the alien theme. Great humor and color DMD adds a lot. Fast with flow. Easier to shop.
Really cannot miss with either, but given the context where both your games have depth and epic depth, I'd might go for MB.
Dan