I don´t know RTR, and I don´t recommend starting a game with a modded one, for by and large they tend to be harder than the vanilla game, according to my experience.
In the vanilla game the Romans would be my recommendation, especially when it comes to the battles. The factions depending on Phalanx units need quite a bit of time to learn how to use properly, that combined with the crappy units you get at a campaign start will most likely be pretty frustrating for a beginner.
Personally, I´d suggest the Julii for a beginner, their units can stand up to the barbarian ones they´ll encounter easily enough, so they´re good for getting a feel for the game without too much frustration. It worked for me, and I´ve started my Total War experience with RTW.