Depends on where you live and the size of your room - my housemate has BX8a's in an untreated room on his desk without any decoupling (e.g. Auralex MoPads, Primacoustic Recoil Stabilizers, etc.), and the low end is honestly quite overpowering, not to mention the fact that if you have ANY neighbor issues, then you can guarantee the bass from those suckers is gonna go right through your walls/floor/ceiling.
SO, if you have any of those issues, I'd go HS50's, but if not, then BX8a's!