u know u maxed vorpal blade out before lvl 9? jsut pointing out. And BANSHEE's veil...

Also banshees will prevent damage from an AOE spell that will infact hit you. No one focuses tanks with or without banshees so that's a dumb reason not to get it

Who cares about speed? U have SD. I would build banshees bcs u get hp and mr.
Fon you get more mr, no hp, hp regen but when will u need that as shen? When u run from a team fight and leave ur team for dead? That's just imo

like sucho said as a tank u dont need to block a spell.
if u compare now the 2 items u get 50mr from banshees and 375 health.
but from FoN u get 70mr and 8%speed.
life reg and mana isnt mentioned here.
on that way FoN is more usefull for a tank

you dont care about life regen as a tank because end game you b when you get low hp.
you get good enough magic resist from banshees and you get hp boost. you waste the mana you would get but the bonus banshee gives is worth it. Force of Nature is for HP regen champs and shen imo isnt one

Yeah blocking one spell will discourage mages from casting on you, but as a tank your purpose is to eat spells you don't want casted on your carries.

why banshees?
i know its a super item and good against caster but at first it gives mana and shen doesnt have mana. second if you want deff against caster just get force of nature, it will give more mr and also speed and great life reg.