10 Inch Tonearms

OP,
It could be because some tables specifically recommend (due to alignment) either a 9" arm, or 10" arm.

I've used 9, 10, & 12" arms (different manufacturers) but was waiting for someone to chime in who used various lengths of the same arm on the same table, (which noromance did) to note their differences.

FWIW, Of the 9, 10, & 12" arms I've used, I prefer the 12" for the reasons noromance stated, but it's really apples/oranges due to the fact that they are not the same arm (arm geometry will make more of a difference to SQ than arm length). Whatever length arm you decide upon, being a perfectionist with cartridge alignment is very important to overall SQ.   

boxer12, do you think adjusting the stylus just in front of the alignment point versus towards the back of the alignment point would offer any noticeable difference?

Goofyfoot,
Yes, try to get it as close as possible (preferably on) to the point(s). Also align the vertical lines (of your alignment gage) parallel with the front of the cartridge while you are doing this. You can hear slight distortion (depending on the cut of your diamond) when off. 

Raul's link at the beginning f this thread explains everything nicely. Tonearms are very simple mechanical devices and not hard to understand. At the bottom of it all is the cartridges amazing sensitivity to vibration. This is where a leap of understanding is required. Just a few angstroms movement (miles as far as an electron is concerned) will create an audible signal. It does not matter what is vibrating, the stylus, the record, the cartridge or the tonearm. A signal will be produced that may or may not represent the music. This is way more significant than a degree or two of tracking error. Spurious vibrations are also not easy to deal with. Frequently you can not stop them. You have to be happy just moving them out of the audio band where they do no harm. 

IMHO it is more important initially to analyze the construction of a tonearm before listening to it. It has to meet certain requirements to be a good arm. I am just as susceptible to visual cues as anyone and I might initially think a cool looking arm sounds fine only to discover after a time it does not. In my experience the arms that failed did not meet all of those requirements. 
I think Marc Gomez's article points to those requirements. I would also add that people without vacuum clamping need to look at neutral balance arms and arms with their vertical bearings at record height. This makes warps less audible and allows the arm to track them better. If a record is perfectly flat it does not matter. Judging by the design of Mr Gomez's arms, he expects records to be perfectly flat or everyone to be using vacuum clamping. 

Goofy, the whole issue of tonearm effective length is less critical, if you are going to mount a mono cartridge (not totally irrelevant but much less relevant).  So use whatever length best meets the mounting requirements of your turntable.