I would actually take Sancho over the other 3 players any day of the week.
Sounds like a bad decision but then you remember this:
Alonso has superior stats to Chilwell and Emerson is at the same level as Chilwell anyway so that one wouldn't be a big upgrade at all.
Isco hasn't done anything this season and would only disrupt the current system where Mount and Pulisic are playing really well, RLC is coming back next year as well so there's no real panic for this position.
Werner...He would be great though, an amazing signing if we could do it.
So for me it would come down to who is a bigger upgrade for the squad on the long run, Sancho or Werner. Hard question, before you even try to answer it, you would need to figure out what role you would like for Werner to have. Is he a partner with Tammy, or his backup? I would say that every big team has 1 main striker and not 2, so Werner would be a backup, we really need a second good striker, but it doesn't need to be Werner.
So yea, Sancho would be my choice.