[stormcloud]

single blade
+3 feats in dueling so
+3/+3 bonus

twin blades
-6/-4 penalty to double weapons but
+3 feats in two weapon fighting negates it to 0/0
extra attack per round
two extra slots for crystal -- 4 total

double blade
-6/-4 penalty to double weapons but
+3 feats in two weapon fighting negates it to 0/0
extra attack per round
two slots for crystals -- but this doubles the effect of a very rare crystal like solari etc.

i've heard you should either be a single blade spec (w dueling)
or
twin/double specialist with master flurry (3 attacks per round)