Tone arm length

@OP. One logistical issue to consider is that the 12inch arm has a bigger footprint due to the longer cantilever mounting plate

As regards sound quality, be sure to consider the Professional PSU. Versus the standard wall wart PSU, this will make a much bigger difference than any differences between the two arm lengths.

The Universal is a superb arm and you won't feel short changed in sound quality which ever length you specify.

@OP There is a 24 volt version of the Professional PSU for use with the Innovation series of turntables. Hi Fi news found that the Professional PSU significantly (measurably) reduced rumble - so it's not just about speed stability. However, the motor itself and the optical control circuit will benefit from a more stable, lower noise supply.  The deck used with the professional PSU has blacker backgrounds, better low level linearity and more articulate bass.