Interesting. I think any difference in performance is probably more to do with the hardness of the dampers rather than the single 'floating' shaft vs dual spindle shaft design. The main advantage of the dual shaft design is that the spindles are controlled in what direction they can move by their pivot (up and down only). This allows softer dampers to be use while still preventing unwanted movement of the spindle shafts. Some heads ( Henseleit TDR and SoXos Strike 7 for example) have a single shaft but have a 'see-saw' pivot in the middle that the shaft passes through, this achieves the same design goal as the dual shaft design.
The conventional 'floating' spindle shaft that is free to move in all directions works just fine as long as quite hard dampers are used so as to keep it's movement under control. I suspect that the dampers supplied with the single floating type shaft will be harder than the ones used with the dual shafts (check them and see if this is the case).
- Soft dampers: Reduced vibrations, smoother response and reduce oscillation at low headspeeds
- Hard dampers: Better at higher headspeed and give more immediate and aggressive cyclic response.
Some heads these days have adjustable damping so you can tune to your preference. Sadly my 'dumb thumbs' are not sophisticated enough to notice the difference unless the change is drastic.
To answer your final question, if it were me I'd try the dual shafts with softer dampers first. It should be possible to fit the hard dampers to the dual shafts anyway if you wanted to try them out. Other than saving parts count / complexity I dont really see an actual advantage in the single floating shaft in itself.