Wind stance, in ideal situations, out DPSes Sun.
The only time it doesn't is against high AC targets, though often with ship/party buffs your to-hit can get pretty high anyway. Auto-crit mobs obviously favour Wind a lot more than they favour Sun.
So really, Wind is best in any non-Epic, against any stunned/held mob, and against Epic Velah.
And Sun is better in the eADQ2 fight, and against the djinns in eVON6.
I would suggest building for the content you run most often, since neither is the "max DPS" stance anymore.
The other thing to consider is that the strikes you get from the Wind stance work against devils, while the strikes from Sun do not. This can be an issue if you're tight on APs (and thus can't get the Void line), as a Horc Ninja Spy would be.
Though, I also agree with this argument.
That's what I remember reading as well.