Scion is a very limited window of character types, so obviously picking an ally wouldn't be them as far as generally speaking. A villager ally for example, would jump through a lot more hoops and be a hell of a lot more loyal than some evil power-mongering demon using darkie-type.
Scion ally over Tribunal? Tribunals can get you unwanted, or not wanted at all.. Trib more useful.
Scion ally over Village? Villagers dont tend to backstab anyone unless its in a specific role-play
or you blatantly use magic/help mages
Scion ally over Outlander? Outlanders have a much more utilitarian benefit than a Scion with a
merc on call, does.
Scion ally over Maran? Enough said.
Scion ally over Empire... That's a toughy
Scion would be at the bottom of MY list for picking allies, untrustworthy and most likely to stab me in the back during their quest for power.