If it is for the Warpriest, you don't even need the Shield Spikes.
I'm not sure on that. Unless the shield itself can be given weapon enchantments, it can't be given the Agile enchantment. I know that in 3.5 you could only give the spikes on a shield weapon enchantments, not the shield itself, but they had separate enchantments. (This was abused when using the 'Defending' enchantment and what-not.) If that's been changed in Pathfinder (I honestly don't know if it is), then you're right; otherwise, shield spikes are the way to go.
...Shield spikes would probably be the way to go anyway, since you could have separate enchantments for the weapon and the shield itself. It's a more expensive route, but it allows you to have all sorts of enchantments if you have the money for it.