Hi there!

Choosing a faction hey?

Well I love all factions in RTW, they're unique in many ways.
In order to choose which faction to play next (cuz 1 campaign takes a long period of time and I definitely want to try all factions) heres what I thought:
There are roughly 4-5 cultures in RTW: Roman, greek, barbarian, "carthage style" and easterners.
So firstly to play as one of the roman families is a must cuz the game is about ROME total war, right?
Then I choose all playable factions after that unplayable (but unlockable)
I choose according to culture, cuz I wanna spice things up. So after I played as romans next I play as one of eastern factions (e.g. Parthia), after that I go north with barbarians (e.g. Germans), then Carthage, after that again one of roman or greek factions and so on.

So this is how I do: firstly try out the most popular factions like romans,seleucids,carthage,egypt,germania, then go for the little ones like armenia, trace etc.
Just my opinion though, what you choose is up to you m8! Good hunting!