Another distinction I'd like to highlight and develop concepts around would be religion and its practices.
The defining difference between Memnos and Thamor, in my mind, has always been this: Thamor wants to see humans inherit the world/rise to power in a society of law, justice, and good; Memnos wants to see humans inherit the world/rise to power but believes that good is superfluous and ultimately a weakness, that true law and justice require ruthlessness, and that the society that ultimately comes to rule the world (that is, his) will do so because it is able to dominate its enemies totally. Thamor is definitely a god not to be trifled with, but he is a god you love and adore. Memnos is definitely a god not to be trifled with, and he is a god you fear and abject yourself to. At a very basic, stereotypical level, I see Thamor as Odin from the Marvel Comics, minus the temper tantrums, with Morgan Freeman's voice, and King Arthur's code of honor. On a similar level, I see Memnos as Zeus without a sense of humor, the need to sleep with mortals all the time, and the ruthlessness of his father, Cronus.
These concepts inform my views of the Sorans and the Parthans. Sorans go to war and are great warriors, but they don't do so because they think war is a good thing. They think that war can instill good characteristics in a man (courage, discipline, honor, comraderie), but war is ultimately the means by which they fight evil, unjust things. Sorans don't go to war for political reasons. They go to war against rampaging Dragons, orks bent on pillaging, Parthans looking to conquer Verosia, etc. Parthans also go to war and are great warriors, but they probably do see war as a good thing. To them, it's sanctioned by their gods and rulers. It's the means by which they will dominate other humans and use them to forge their great empire. With that, they will conquer the world and see the race of the Duneimen put down the threats of the monstrous humanoids, the Giants, the Titans, etc. Parthans, on a basic level, ritualize violence and admire one's prowess in such fields. They kill animals for veneration. They enjoy gladiator games (although I imagine them ritualizing them as well, and not treating them like base blood sports as the Romans did). When they duel, they do so to formally murder one another under sanctioned settings as opposed to the noble jousts of the Sorans.
Moving on.
I like to imagine the Sorans as being Judeo-Christian in their outlook, with some caveats. Obviously, they are a polytheistic bunch. On the other hand, again, I like the idea that Thamor is a god you love rather than fear, and that this holds true with many of the Ronan Pantheon deities (the exceptions being obvious: Kalrik, the goddess of death, etc.). While the priesthood is a (more or less) higher class institution (in the sense that, once you become a priest you are of higher status than a serf), I like to think that they are still approachable. Their priests serve as chaplains to common foot-soldiers, they minister to commoners, aid the poor and sick, provide spiritual advice, etc. The idea of it all would be that church (kind of like in modern-times Christianity) encompasses the community and is part of it at the same time.
I like to think of the Parthans as subscribing to the Greco-Roman mode of religious mysteries versus religious community. That is, the priests commune with the deities, lead ceremonies, deliver omens, etc., but the common people are often kept separate from much of the process and are left to their own humble prayers while waiting for "the verdict". Parthan congregations would be for specific holy days, but outside of those occasions (and other times the priests summon the people), the Parthan Order is inaccessible and veiled behind an aura of mystery, power, and fear.
As such, I see Parthans resorting to quasi-ancestor worship for security and comfort, since most of their gods (or, at any rate, the customs of the Order) are not necessarily geared toward that. I also see disparate, more-or-less informal hero-cults among warriors, to supplement the official worship with the more day-to-day spiritual reinforcement that warriors often need. These practices center around crude icons, relics, statuettes, etc., but never rival the Order itself. The long-dead heroes themselves are revered as exemplars of a god's facet or even as servants of said god, to ensure the kind of censure contradicting the Order might bring about.
Bottom line? In Sora, you can go to your priest and talk about what ails you - physically or spiritually. You might have to wait your turn, but you'll get some spiritual guidance. In Parthus, you might have to travel to some craggy peak or some deep cave, or some dark wood, or into the sanctum of the Primus' citadel. And you better have a good reason for doing so.
I imagine religion determining law and custom in many cases. These should stand in great contrast to Rona and Sora's. One thing that blew me away when watching "Rome" was an episode where the main character - a Centurion raised to the status of Equites (knight) - is able to buy these fine lands for his family. His wife and he are expected to "consecrate" the lands in a pagan ceremony to Mars, in front of a priest and other witnesses. I'm not advocating pornography or risque sexual material by any means, but pagan, almost elemental practices like that - animal sacrifices, consulting oracles in foreboding temples - should be the difference between the two realms.
For instance, the idea that no Parthan can enter a Parthan community in armour outside of times of war. War is a formal, ritual thing, and must be approved and declared by the Order. Only when the warriors have been consecrated can they walk in the guise of war through Parthan lands. Because it's so serious a matter, perhaps the decorations that make Parthan armor unique are themselves religious or symbolic in nature. Red might be the color of blood and war, and thus worn only when one expects battle (it might also be worn when you wish to advertise that you are seeking mortal combat with someone). Purple is the color of rulers (Memnos) and their immediate companions, and thus the right to wear such clothing rests with the Primi and their "Centurions". Or, for another example, non-Parthans cannot walk with unbound weapons within Parthan communities.
Communal feasts (perhaps an end-of-workweek celebration?) might themselves be a religious affair, with the event starting with the ritual slaughter of the animals to be served, the dedication of choice parts to the gods, auguries performed from their entrails or offal, etc.
The bottom line is this… I want the Parthans to be a pious people, a god-fearing people. I want them to adhere to customs and laws that enable them to rightfully feel like they are a just, lawful people… but I also want them to be a ruthless people at the same time.
As their "age of heroes" rises, these themes will obviously clash with the adventuring, free-wheeling spirit of the younger generations. This won't necessarily be a paradox, though, since the "heroic" ideal of the Parthans will be more like the Homeric one (where heroes were still god-fearing, if occasionally reckless and plagued by hubris). Thus they will still be willing to observe most such traditions (if not the ones that bind their behavior - like when to eat what, when to drink what, how many concubines to have, etc.).
Thoughts?