The Illusion of Choice

[Reynhart]

The "best" option here is probably Rage of Halone, since it has a higher enmity modifier than Savage Blade (at least, as far as I recall, it does). This would allow you more Royal Authority combos while tanking since you wouldn't need as many Rage combos to hold hate. The best option for OT is probably the Fast Blade one.

But, when doing Royal Authority, you don't use Rage Of Halone, so your additionnal potency is not used.
As for Fast Blade, unless you're OT all the time, it's the less effective boost since it only adds potency. But if you tend to spend a fair amount of time in Shield Oath, or are generally better geared than the average DPS, the enmity boost wouldn't be important.
Savage Blade could also be a good choice since it adds a little bit of enmity, and is on the path to RoH and RA, useful as a MT and as an OT.
Back to Rage Of Halone, you could also take critical hit into account. If you crit on a Fast Blade, you only apply the crit bonus for this 10 potency boost. If you crit on RoH, you could apply it on the full 30 potency.

And what if I added "Increase Riot Blade's potency by 30" ? Still the same overall increase for a 9-GCD rotation, but if you tend to use a lot of MP, you could end up with a higher (Or, should I say, a less low) potency for when you focus on refilling your MP.

You nailed one thing though. It's the OT vs MT. There could be traits that only a MT would use, like an enmity boost, or the ability to block magic, or traits that only an OT would use, like an increased DoT potency for Goring Blade when performed from the back. These two lasts traits could be a good start to make two PLD not really the same.