I have to disagree here. The level of pain doesn't vary at all, since he took out every bit of damage, exhaustion, pain out of luffy. Thats why luffy was able to move so lightly after he woke up. Sanji simply wouldn't have survived it. Even nami says that zoro is the toughest crew member out of all members. If Sanji would have survived due to plot Oda would have chosen Sanji to take on luffys damage together with zoro. There was no way for Sanji to survive it, Oda knew it, Zoro knew it. Zoro even himself wasn't sure to survive that, but he'd rather sacrifice himself than let one of his best friends die. Thats why he knocked out Sanji. That was a clear statement of Oda that Sanji would have died for sure if he took on that instead of Zoro.
Zoro taking it means that Sanji had no "plot armor."
And narratively speaking, Zoro's tradition is performing while compromised. Which made that such a big deal. Mihawk and Kuma were impossible to beat pre timeskip for any Straw Hat, so they were able to push him that far. Sanji pre timeskip couldn't beat Gin "due to damage from Pearl" or post timeskip can't beat a Doflamingo subordinate "due to damage from Caesar" so there's your difference.
Look at it like this sanji took way more damge than zoro during thriller bark & yet he still was able to get up from kuma shock wave along with zoro only when zoro took luffy pain his damage surpass sanji but I have to because of thea amount of damage sanji during all those fights plus kuma attack I don't think he would have been able to take luffy's pain & I guess zoto knew this; that's why he was able to knock sanji out but zoro could have taking it because he relatively was fine during thriller with much less damage but if sanji did take as much damage he would be able to take luffy pain