Uh.. correct me, but it's pretty hard to reach 320 ATK with a (more or less) normal Sin...
My crit sin right now (lvl 93) has around 50-60 Base LUK (to reach 48 crit), 99 Base AGI (yeah, I have no flee gears, my flee is at 250~ -_-), and 60 Base Str. Rest on 1+??.
Damage is around 300-400 each hit (added the high and small number together), with 186 ASPD.
It's not much, but oh well, I don't have Kahos, and really use nub gears on that Sin (Poring Card FTW!). At least the speed is nice.
I would rather suggest a DDSA. But that's just an opinion from a nub. o_o
I really doubt there will be ZOMG SO MUCH DIFFERENCE between +65 ATK and +20% damage...
EDIT: Oh well.. better read first, then answer.
SinX. Uh.. prolly has more STR? Argh dunno, DDSA will probably be better at low levels.