Hi Lindsay,
I have just changed my turntable to a Kuzma Stabi R with a Viv Labs Rigid Float 9" arm and an Ortofon SPU Royal N, suffice to say I am very happy.
If you speak to Kevin at Definitive Audio he would tell you that whilst the Stabi M is marginally better, most people chose the Stabi R as they find it very difficult to justify the £10,000 extra on such a marginal gain.
I listened to the 4point 9", Rigid Float 9" and Stogi 313 reference, using an Ortofon SPU Royal N. The 4point is very punchy and dynamic, the 313 sounded matter of fact, a bit boring really, the RF was just as punchy and dynamic as the 4point but was more organic, it just seemed to let the music flow a bit more, the icing on the cake is that I got the RF 9" for a good price as he is rationalizing his stock.
Kevin has all the facilities to listen to all the 4point arms, Rigid Floats with 3 Stabi R's at his disposal. His cartridge of choice will be the SPU Royal N and at £1250 it is a veritable bargain, he told me that it is his favourite cartridge full stop, now considering that is comparing to Kondo's, Fuuga, VDH's etc etc.
The £10,000 you save on the Stabi R vice the M would allow you to put second arm say a 4point 14" and cartridge on the Stabi R, the world is your oyster.
Hope this is of help.
Best regards, Alan