Counter target spell if you control more creatures than that spell's controller.
"We're glad to be of service—to be an audience to your failure." —Maizah Shere, Tazeem lullmage
Official Rulings for Unified Will
6/15/2010 : Whether you control more creatures than the targeted spell's controller is checked only as Unified Will resolves. If that player controls more creatures than you at this time, or you each control the same amount, Unified Will simply doesn't do anything.