Personally I would pretty much always choose Banshee. It gives some anti-harassment/ anti cc . It helps with an initiation. health and mana
It actually gives more magic health (health you have against magical damage) then FoN.
For those that don't understand how cause FoN has more magic resist, I''l try to explain it as simply as i can. The more health you have the more effective magic resit/armor are, since Banshee gives you both its more effective than FoN