You need to keep in mind that if u`re using a 1 handed weapon u get to use a sigil which gives 5% more m.atk and some p.def.
As for the +16(not +15) weapon question here is the answer: http://boards.lineage2.com/showthread.php?t=248755
keep in mind those weapons are blessed.
But the difference between 1handed + sigil and 2 handed isnt that big, so some prefer the extra p.def, its all about playstyle.
Wow that is some impressive and weird testing hahah thank you for the link.
For dual class an enhanced shadow retributer is enough imo. You can do all dailies with it. Apoc +17 is better indeed but do you really wanna have such an expensive piece for a dual class? As for R99, worse than apoc +17 (compare matk).
Or what exactly do you wanna do with your dual? Solo grinding? Then you will need a weapon as op as you can get.