Truth be told, I'm at a loss too. Yes, you said "If you time it properly then SBlock is your dump, if you cant guarantee a SS during block, then I would HS." But the first part of your post made it sound like waiting for SS to come out CD before using Sblock is a hard thing to do while offtanking.
Anyways, at 10k vengeance or so, depending on you base str and the weapon you use, 0.75*Sslam equals 2*HS. After that the difference keeps growing and growing.