What's the + and - of the 12" Tonearms

The greater length reduces stylus tracing error, which is a good thing. For pivoted arms, the stylus is aligned perfectly in the groove (i.e., perfectly tangential to the groove the way the cutting stylus cut it) in only two places in its travel across the record. The longer tonearm reduces the amount by which the alignment is off everywhere else. The tradeoff is greater mass and the need for a larger plinth or arm board on which to mount the longer arm.
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If you have the room for a 12" arm, and a removable armboard, you can get a good idea of how well one of these arms would work for you by investing $200 in the one sold here on Audiogon (search by "Cherry Tonearm." I bought one of these on a whim and prefer it to my VPI JMW 10.5, although how LONG I prefer it may also depend on a whim! In any event, it's quite the bargain, IMHO. Dave.

Issue 51 of HI FI+ features a comparison of three 12"arms the Vpi 12.7,Kuzuma stogi 313 and the Schroeder reference.Roy Gregory talks about the theory longer arm lower tracing error and the bad things higher cost,higher effective mass and inertia and turntable compatibility [not mountable to most closed sprung chassis tables designed for 9" arms]His summing statement is that he could live happily with any of the test arms and based on the evidence a good 12" will always beat a good 9" and that they have an ease and natural sense of musical progression and they help remove the system itself from that process,Issue 50 tested a SME20/12 and 52 a project p9 carbon 12"arm and the Brinkmann.Its worth a read.