definitely not knife[4] guessing you want to melee with daggers? read the sinx discussion/search forums
I'd recommend +10 main gauche[4] (same slots, more atk-if its not +10 it'l be hard to resell cuz its pretty easy to find/make)
it does depend on your build. you are correct about DDPT(if str is high)
or +8-10 DDP glad(no need for DDPSamurai mg)
for the other hand...I'd have to ask you what you are using it for.
USUALLY for main 2 weps u dont need another 4 slotted wep, instead, a +8-10 gladius/sabre is used.