The Romans were very good and disciplined. However they needed direction. Once their leaders fell the unit would be lost until another officer stepped in .they historically had issues with adapting to new battle scenarios . They paid for that shortcoming against the guerrilla tactics of the Gauls.
The Spartans on the other hand were trained not only to work as a group but more so to protect each other . They were also skilled in individual combat . They were not so tied to their leaders as they were to the battle objective.
I'm leaning towards the roman, mainly due to tech, steel and Iron against bronze... would be an interesting fight to see... where is Q when you need him.