Given the choice in the title, 6mm or 9mm, I'd say, go with the 9mm.
Both sizes have options for balsa, meerschaum or charcoal filters. Both options have adapters for using the pipe without a filter.
But, when choosing to use a charcoal or meerschaum in-line filter, the draw on the 6mm is too restricted (for me, anyway). Whereas the draw with 9mm filters feel more open and effortless. I do not, however, find there to be much difference in the draw with the balsa inserts.
Of course, this is just my opinion.