all other things being equal, a 12" arm will certainly result in less tracking angle error than a 9" arm. but whether or not a specific 12" arm is better all-around than a specific 12" arm - well, there are a lot more wariables inwolved. each arm must be judged on its own merits, imo. many of the most highly-regarded arms are 9". arms made by sme, rega, wheaton, kuzma, & wilson benesch are some that come to mind...
doug s.