Yeah, not trying to scare you. I hope Dunham wins. But if not for Sherk's injuries and the fact that he had been out for a while, I would favor him.
Dunham's all around bjj is better for sure. But he is not going to submit Sherk. Sherk has some very solid guard passing skills (see Hermes Franca). I would put Sherk's wrestling ahead of Dunham's though, we disagree there.
He has only lost to champions (Edgar, Penn, Hughes and GSP). There is no shame in any of those losses. I think you are underestimating Sherk. I like Dunham too, but I think a healthy Sherk would beat him.
That said, I hope Dunham wins and I am not trying to scare you out of your bet. Liek Ludo said, this is a forum where we talk about our bets and why we should or should not make them.
I think Dunham is overvalued even for a regular sized bet. But Sherk will have been out a 16 months with injuries... this is too competitive a fight to come back to, and I think Dunham will get it done. But I would put him at like -130.