Tone arm length

A 12" arm will not resonate more simply because it's longer, it certainly will resonate at a lower frequency. The amount of resonance is far more a function of materials and design. Also a longer arm is easier to set up because it has lower tracing error, and thus minor misalignments are less noticeable, the same way that minor VTA changes are less noticeable. Typically a long arm will have higher mass and so favor lower compliance e.g. moving coil cartridges. And all of those things together enable high performance stylus designs like Shibata, Fineline, Microline, MicroRidge, et al to perform optimally. 

Personally, I find that the 10-11" length to be the best compromise. Like the VPI JMW Memorial 10.5. With its 3D printed epoxy arm it is well damped, long enough to enjoy lower tracing and  VTA errors, and I don't ind it being a unipivot - they do not bind or have bearing chatter, maintenance is easy, and swapping arms takes about 30 seconds. OK, the second arm tube isn't cheap. But it sounds great, and puts the fear of God into people who shouldn't be using it anyway, e.g. party guests.

From 1970 to almost 1980 including all of my college years, I worked in hifi retail stores first in Seattle and after graduation in San Diego.  The stores I worked at were primarily McIntosh dealers and at that time a great deal of emphasis was placed on clinics including turntable and cartridge setup.  Every dealer was required to have a very expensive bifocal microscope (Wild-HerrBrugg) specified by McIntosh that was actually capable of seeing if the stylus was mounted squarely on the cantilever; seeing if the stylus was wearing evenly over time and so forth.  Customers were encouraged to bring their TTs in for periodic inspection.  Some of us who took an interest were trained and became proficient at TT setup and evaluation.  I still own two of those microscopes.  Over time I became convinced that all things considered the 9" tonearm was the better option.  This is not always true, but it is so more often than not.

@billstevenson

Interesting and thank you for sharing the insight. By any chance did you ever hop on the ferry over to Victoria and if so, did you visit Sound Hounds?   

Interested in selling one of those scopes???

btw I've bought the TT with the 12" so that's that.

My wife and I enjoy Victoria very much and try to visit there every chance we can.  Perhaps our favorite city.  I will try to visit Sound Hounds next trip.  I am quite sure you will enjoy your new set up.

I almost forgot about the microscope.  We are summering at our home in New Hampshire.  The microscopes are both at our winter home in Florida.  If you are really interested send me a private note and when we get back in late September or thereabouts I'll send you photos and we'll take it from there.