It is true that the longer tonearms are living a second life now. More and more listeners are intrigued by this longer arms.
I don't have any experience at all except one with 12 inch arms. But with the Moerch DP8 trying the different flavour is just a question of changing the arm tube and counter weights...and of course having a turntable that allows you to mount tonearms of this dimension. I had a very busy month in doing this since I really wanted to get to something real and not a maybe. The difficulty of setting 12 inch arms is that we have much less margin of allineament error. If we are not spot on, we could easily loose its advantage. To be as precise as possible I used a digital microscope so that for the difference in sound the set up could not be held as responsable. I used also the same tube mass but it is true that adding more counterweights alter the mass of the tonearm, so at the end I just set up each respective length in the way it was performing the best. For the listening I have used these turntables: Garrard 301, Thorens TD 124, Platine Verdier, Brinkmann Balance, Nottingham Analogue Dais. The difference between the 9 and the 12 were revealed in each set up with any system I have tried them. Since these turntable don't belongs to me (except the Garrard 301) I listened in different systems belonging to my friends or customers. I had to do this since the other turntable I own (Linn LP12 and Oracle Delphi ) don't allow the mounting of a 12" tonearm.
The work has been long, but the outcome not too difficult, much easier to detect and to appreciate the differences then I would have expected.
It is not going to be a long writing since I can summarize it easily.
There is not a better length when the two are correctly set up. The 9" is apparently more dettailed in the mid -high, high frequencies. I say apparently since I have not really understood what happens. At the beggining I though it was more dettailed, then I thought the 9 version added false dettails! I realized it highligths details in this frequence range. But I am aware also of some details that with the 12 I am not. At the very, very end I have a feeling that the 9 length more dettail (compared to master ADAT or tape I had of the same LP played) is the result of a slight inaccurate note decay. Go figure that out...
This medium high frequence range is not just brighter, it is faster. it is like the younger version of the 12inch. It has less timbre saturation and I believe for this reason the leading edges stand out better and with this we have the perception of a better ritmic propulsion. It is not more ritmic of the 12 version it is just that this aspect of the music is more evident. The 12 inch is more "organic" every parts of the musical message are blended togheter with a lesser sense of having a different origin. The 12 inch is calmer, more contemplative, bigger sounding, the silences are more profund and the bass surely goes more down and is more natural in its development. But it is less... awake. It has less tension, but tension used as a positive term. If the 9inch is a 100m sprinter the 12inch is a 400m runner. It is more condifident but less passioned about its playing. The 12 emotions are more controlled then the 9 inch version.
The bad thing is that it is not a question of liking one better then another. In some turntables like the Garrard the 12 inch is better. It copes better with the design weakness and qualities. In the Brinkmann Balance... I liked the 9" better. The 12" made everything sound more distant, with less soul. Too much of same good things. With the Nottingham Analogue Dais and the Platine Verdier... It is more complicated. You are sure the 12 is going to sound better; you are sure it sounds better after switching it for the 9" but when you put back the 9" you find it better... I guess in this case, with this kind of design the length is not the dominant factor. The Thorens 124? Very strangely and unespectedly it was better with the 9". I thought that with all the noise cause by the direct drive the longer tonearm mounted more distant from the turntable chassis, (a special armboard has to be made for it) would have sounded better like it did with the Garrad 301. Not in this case.
My conclusion after all this work is simple: this revival of the 12 inch is justified only because there are turntables designs that works better with the 12 inch version and not because the 12 inch length is better then the 9" one. So don't overview a turntable just because it cannot accept the 12 inch tonearm and for the same reason don't buy one because it can accept a 12" tonearm.
The bad thing is that even with a turntable that can accept both lengths you could prefer one over the other. At the end it is very annoying. No final result. None is better then the other. Like with everything else you should have both length to decide. In my listening session even 12 inch tonearm only forever supporters had to surrend to the evidence that with certain turntables the 9 inch length was a better match.