I'd wish I could go with Sif as she's has better speed and agility and is strong as well(30 ton). I would if she had a sword that had some magical properties that allowed her to pierce his skin. Without that, I really don't think she can do much damage to Thing. I also think it will be hard to for Thing to even touch Sif with her speed and skill.
Thing started out as a sub 50 ton guy, but now he's up closer to a 100 ton guy. In the end, the Thing would thunderclap and stun her and then nail her on the chin and KO her.
Sif is Faster. Sif heals faster. sigf is immortal. sif does not tire normally. sif has better agility. Sif is incredibly hard to Kill due to her immortality. Thor has incredible pain resitence due to being an asgardian. and her sword has cut far tougher.
if she has her sword she can take him.
Honestly I don't think a sword is the weapon that Sif will use to lay Ben low. It's gonna be those big blue eyes and slightly pouty lips.
Unless Sif goes stark raving pyscho, or he's Mind Controlled by someone, I think ole Bashful Benji is going to be too much of a gentleman to K.O. Sif with a Sunday Punch. So I say she gets him to let his guard down and she cheap shot him, and Thor and Hercules spend the next 6 months teasing Ben /mercilessly/ on Poker night.