Glad to hear there is experimentation with this concept!
The difference in cost vs. The NrT towers would be the tough choice.
That would be dependent on each persons situation. Having the room for the monitors but not towers..........RAAL vs NrT.......etc.
Space for large speakers is not an issue for me, but cost is. If I could manage $2000 for the towers I'd be sticking with the excellent NrT tweeter. If I wanted the goodness of the ribbon combined with a little better midrange over the Sierra-1 NrT then a Sierra-1R(ribbon) crossed over to my 12" Rythmik would fill the bill.
Tough decision.