Soo when Action is talking about who to give limit break too.. Rydia may do more damage based on Turns, but they are LONG turns.... As casting Dualcast Meteor does add up. Dualcast Meteor is not long at all, especially once coupled with Fast Talker, which puts Rydia way ahead of damage. The problem is simply that Dualcast Meteor burns MP quickly too, which means one character will be losing turns in order to help her. This point is entirely negligible and is based on personal preference on whether you are willing to allow a character to do that.
What Action is saying is the damage potential that Kain would lose if he didn't have Limit Break, since he and Cecil are the only ones who can hit 99,999 damage. Potential is the key word here, Rydia gets about a 5000 x2 more damage with Limit Break on, but Kain gets 90,000 more damage with it. Compare and contrast the difference. So while Rydia can do more damage in the long run, Kain can get the most use out of it. This, also, is simply a matter of preference on whether you prefer speed or if you prefer putting Limit Break to its full use.