9", 10" or 12" tonearm?

@dazzdax i also use "9, "10 and "12 tonearms and all of them are great, but for some turntables like Technics SP-10mk2, SP-20 the "10 and "12 tonearms are much easy to use and to mount. Aesthetically "10 and "12 are better (imo). So you can't be wrong with "10 inch tonearms like Technics EPA-100mk2 for example. My reference is Reed 3p "12, it's had to find anything like that, because with Reed anyone could adjust absolutely everything (including azimuth) on the fly (the best engineering).  

Another advantage of a 12" arm is that once you set the correct VTA/SRA, it changes less with different weight records than does a shorter arm.

Another advantage of the 12" arm that has not yet been mentioned is that VTA changes less with different record thicknesses.  Also, because the offset angle is lower on a 12" arm, skating forces should be reduced.  

As others have mentioned, the tradeoff on long arms is that, for any given effective mass, the arm will be less rigid and more prone to problems with resonance.  Not only are the arms themselves slightly heavier, having the heavy cartridge well out on the end of what is a cantilever means that the effective mass (inertial mass) of the cartridge is much higher.  If one were to try to reduce effective mass by reducing the mass of the arm itself, that means less material in the arm and less rigidity or damping capability.

Linear tracking arms come in a WIDE variety of designs.  Those that rely on very low friction to allow the arm to be pulled along by the forces on the stylus (like the air bearing arms), put more stress on the cartridge's cantilever than do pivoting arms (because of no mechanical advantage of a pivot and lever).  Some say that the requirement of such designs to yank the cantilever sideways enough to move the arm actually means the cantilever is almost always further out of tangent than on a conventional arm. 

Whatever the theory, I cannot say that I've heard differences in performance that are consistently attributable to one type of arm or to particular lengths of arms.  

VPI's HW is working on a 14" arm that's nicknamed the Longboy. I got the feeling it ain't goin' to be cheap :-(

http://www.vpiforum.com/viewtopic.php?f=9&t=9547

Clearaudio has a line of linear tracking TAs that are typical CA quality (very high). Even the least expensive is expensive (to me). I'm saving for new TA and may spring for their lowest end model. 

Another problem with linear tracking TAs is their physical layout. Most will not fit under what I would consider a "normal" dustcover.