IG all the way. If you aren't Battle Brothers, the Allied models count as "enemy" models. So your Rune Priest definitely shuts down the
GK like
DaddyWarcrimes says.
Plus you are looking at allies wrong. Yes, Vindicare Assassins are cool. Very fluffy, fun to use on the table. But you don't take Allies just for 1 unit usually. Allies dramatically change how you build your list. You don't just build a
SW list then bolt on a random allied force. You'll want some cohesion.
In the end though, its a game of toy soldiers. Unless you want to win a tournament just do whatever you want. No need to ask opinions about it. The only objective advice people can get is from a competitive standpoint, and I wouldn't be trying to force
GK into a
SW list to get a Vindicare Assassin if I was going to a tournament.