I would think this is simple. The 3.7s are updated and the 20.1s probably will be soon, so for now there are a couple of things the 3.7 *might* do a tad better - but the main quality that the 20.1s have is the push-pull force on the ribbon and quasi-ribbon, so dynamics are a lot better (assuming proper amplification).
Also, they removed the external x-over on 3.7, so I think (I'm about to start a thread on this) that the active bi-amp option is out with 3.7, while it is still available on 20.1, and active bi-amp will generally add to the more dynamic part of the equation for 20.1.
If (this is a big if) you play a test tone, or a single singer who sings in a range not used by the ribbon, and if that singer doesn't have a lot of dynamic swings, then perhaps the new quasi-ribbon will for today out-do the 20.1 which will most probably be updated soon with the quasi ribbon that started on the 1.7. Also, maybe not! Because the push-pull might make the old quasi better sounding than the new one as implemented in the 3.7/1.7. All conjecture.
