If going purely for PVP bonuses, it doesnt matter.
Both SUCK in PVP, Luke more so.
Luke Cage unlocks Premium Mission (10.6), so you should get him.
like 128.135.100.115 said, both suck a LOT in PVP. Anyone who says otherwise never got past diamond mode.
Personally both are good choices and have the highest steady stats( acc, attack, hp) which are what a scrapper needs. I find bleed damage more annoying than counterattack so offensively I'd pick sif. Defensively I'd use luke cage. Playdom may release a new skin for Sif because of Thor 2. Personally I'm hoping Fandral is released. An Asguardian infiltrator would be neat and would wreck Iron Patriot.
I prefer Sif than Luke Cage cause' the costume of Sif makes her good, the only attack I ever preffered from luke cage was the face punch. Which you can only use when level 6 while Sif's Leaping slash is on Level 2 so I obviously prefer Sif