That's just it, you CAN'T land a crit against undead. Thundering will NEVER come into effect against something that you cannot crit. It doesn't matter if it's sonic damage. Read the description, it says 'when you deliver a critical hit...'. And you will never deliver a crit against undead, hence no thundering damage. It's just like icy or flame burst, which would also never work against undead, save the +1d6 elemental damage that goes with every normal hit, not just criticals. If the pick is a must, go with Holy first, then Bane, but only if you are really fighting a lot of undead. Otherwise, just enhance the bonus so that it is more effective against other creatures besides undead. Oh, and Ghost Touch would be VERY useful.