Muti: DV or STS / Spell pen ?

Hey guys.
Recntly I started to play rog for run and its NE ( Nighe Elf ).
So I gotta chose between getting DV / STS for muti and what trink is better?

Also back when I played muti rog on cata I used spell pen ( not huge just like 2 gems in blue socket ). why? cuz muti is all about posion damage ( nature )
so Im wondering if its good / worth doing that on wotlk?

I assume you're talking about PvP, since Spell Penetration is useless in PvE.

I'd definitely go for Sharpened Twilight Scale, as in the end the AP buff you gain is huge. Along with that, your damage isn't pure poison, lots of damage comes also from your auto-attacks and Mutilate, so having <some> ArPen doesn't hurt you one bit. In the end it's the amount of AP you gain that matters.

Just in case you're asking for PvE as well - you go for Sharpened Twilight Scale as well, same reason.