Honestly the best way to learn about the different factions is to play them all - just start recruiting random people from their villages (or start a new game to try them)
saranids have some weird lineups, but their archers upgrade to jezzailis.. not sure where the rest of them go, was never quite interested in any of those (and i hate the khergit/sarranid side of the map because of steppe bandits)
rhodocks have spearmen, stormtroop, crossbowmen, and condoteirre things.
spearmen have board shield and 1h polearm, usually poke people with it.
stormtroop are heavier armored, carry 2h polearm with giant blade on the end ,but no shield
crossbowmen have crossbows. Fairly good with them at higher levels (carries board shield and 1hander as well)
pistoleers are... er... not really sure what they are supposed to be. Carry flintlocks and board shields (and possibly bola?) I generally prefer straight crossbowmen, but they are probably worth bringing some in your line
their cav has some good armor/horse, and THE BEST shield** in the game.. unfortunately, the shield is also purchasable it seems. But they have a minor issue of sometimes spawning sans weapon.
Only myself know much about the vaegir/swadian/rhodocks, been meaning to learn nords though someday.
**reinforced plate shield, I believe.. slightly more resistance than regular plate