I've currently got a single cast ability in my PvP rotation which is Telekinetic Throw which I also have a possibility to proc to reduce cast time by 50%. I've already hit the softcap of willpower so its really between power and surge I would have thought. Am I wrong in thinking the constant extra damage is better than the 8% reduced cast time on a single spell?
It of course depends on your rotation. If you're not using cast or channeled abilities at all, then go power.
On another note I wouldn't necessarily stop stacking a stat just because I hit the soft cap.