10 Inch Tonearms

Most important factors:
Longer arm reduces tracking error.
Shorter arm easier to construct rigidly.
Shorter arm less mass.

The mechanical performance and resonance matching of arm and cartridge are far more important than a tiny reduction in tracking error.
If you don't like tracking error then get a parallel tracking arm.

Very little difference between 9" and 10" arms on any of these factors.
IMO not worth going for 12" but either a 9" or 10" is fine.

@goofyfoot 

I've seen Jelco arms on the used market, some of them 12 inch, some of them 10 inch and some of them 9 inch.

I have a 12" and a 9" Jelco 850 on my slate 401 and swap cartridges between them and can make on the fly comparisons. With that in mind, I can confirm that the 9" is faster and snappier while the 12" is not quite as fast but has a more expansive 3D soundstage and a tad more color which I find more musically satisfying.

Noromance, same cartridge on both of your tonearms?

Goofy, exactly what are your remaining questions about 10” tonearms?

Dear @clearthinker : ""  are far more important than a tiny reduction in tracking error. ""

you are just rigth and you can be sure that kistening ( every thing the same ) OP can't be aware of that tiny tracking error and even maybe he not know what to look for ( during listening tests. ) and be aware on.

Btw, in other words lewm just asked him something that at the end I asked him too before. I don't know for sure but seems to me thahis overall understanding about is low enough to make all his questions or is trolling and nothing more. 
Who can know about but only him, I'm guessing.

Enough for me. It's futile to follow in this useless ( for me. ) thread.

R.

@lewm No. But quickly swapped headshells. Same noted attributes follow the arms to different cartridges.