The thing you need (WoW Gold)to remember is: Frontload damage and speed is the most important thing on your weapon. The stats are always a nice side bonus, but they are just that, a "bonus" secondary to the above mentioned.

Stormherald:
386 - 579 Frontload Damage (average 482.5)
3.8 Speed

Merciless Gladiator's Bonegrinder:
365 - 549 Frontload Damage (average 457)
3.6 Speed

Here's how it works:
Everytime you use your (WoW Gold)Crusader Strike (instant attack), you will use the frontload damage, added to your DPS from attack power multiplied by the instant attack normalized 2h weaponspeed (3.4) to get how much you hit for.

Everytime you proc SoC, you will use the frontload damage, added to your DPS from attack power multiplied by the actual weapon speed (SoC is not weaponspeed normalized!!) * 0.7 (only 70% damage) * 1.1 (assuming you use sanctity aura, which you should as retri).

Lets assume you have 2000 AP semi buffed.
2000 AP/14 = 142.86 DPS
Example: Stormherald:
Crusader Strike: 482.5 + (142.86 * 3.4 ) = You will hit for an average of 968 before armor mitigation.

SoC procs: (482.5 + (142.86 * 3.8)) * 0.7 *1.1 = You will hit for an average of 790 holy damage. Example: Merciless Gladiator's Bonegrinder:
Crusader Strike: 457 + (142.86 * 3.4 ) = You will hit for an average of 943 before armor mitigation.
SoC procs: (457 + (142.86 * 3.6)) * 0.7 *1.1 = You will hit for an average of 748 holy damage.

Conclusion:
While your white hits will average out the same DPS (bigger slower hits vs slightly weaker, slightly faster hits = same):

-Everytime you do a Crusader Strike with Stormherald (6 times a minute) you will do an average 25 more damage than with the Merciless Gladiator's Bonegrinder.

-Everytime you get a SoC proc with Stormherald (~7 times a minute) you will do an average 42 more damage than with the Merciless Gladiator's Bonegrinder.

While an average 25 more damage 6 times a minute and an average 42 more damage ~7 times a minute might not sound like much, it actually is a pretty huge difference in practicality, especially if you consider the following:

Keep in mind that with SoC, the higher your AP the bigger that difference is (due to the slower weapon), I hover around 3k+ AP fully raid buffed, making the difference with SoCs even higher.

Also I didn't factor in all the multipliers that really further increase that difference for both CS and SoC: 15% from Vengeance (should be up all the time), 2% from Improved Sanctity Aura, 30% if you're using (Gold WoW)Avenging Wrath, and 100% more everytime you crit.

This is what makes Stormherald far superior (and Torch of the Damned even better).

Anyways, Merchiless Gladiator's Bonegrinder is also an excellent weapon, but I hope the above answers your question.