In my opinion the biggest "advantage" with the "new bristled brush" (to quote Neato's web site) is the dramatic noise reduction on hard flooring. It picks up from my low-pile carpeting and hard floors basically as well as their first-generation 6-vane beater-only element.
Neato is fairly clear in their phrasing regarding these elements - this is their quote for the bristled brush:
"A new bristled brush improves the pick-up of pet hair, dander and fibers."
This is their quote for the XV-11:
"Neato gets all of your debris, dirt and even tough pet hair into your dirt bin resulting in very low maintenance."
So even Neato felt justified in inserting the word "improves" in advertising the new brush element. This may also be because of the helical design - it makes sense that more debris may be guided towards the intake throat with such a design change - and, let's face it, most traditional vacuums incorporate that design as well.
The interesting part is that nowhere does Neato officially state "experience less motorboat noise on hard floors with our new brush!" That would imply a negative to part of their lineup, and that's not good business practice. On the other hand, as it is programmable, if you're out of the house, who cares how loud it gets?
As to the traditional 6-vane beater, I don't think its inherently weaker per se; it may be an option, though not clearly stated, for those with higher-pile carpeting or rugs, where the brush element may experience issues. One may have to wait to see what the instruction manual states for the consumer when it comes to Neato's recommendations in choosing which element, and for what scenario's, a choice would be made. (If, indeed, the XV-21 ends up coming shipped with both.)
Of course, the undeniable advantage is in the redesigned filter, which is NOT an option on the upcoming XV-21. Again, to quote Neato:
"(The) special filter retains more than three times the amount of fine dust particles of standard filters, which is perfect for those with allergies."
Allergy relief is a big selling point these days, and they have a winner with promoting it this way; however, the improved airflow is more important to me, and is obvious. But "improved airflow" wouldn't necessarily be incorporated into Neato's promotional language, as, again, that implies a negative to part of their lineup.