Well, either way, whatever floats your boat really. Could even have a mix- a soldier that's just fought in Colonial wars, and only just that. I would like to see tension between not just the real veterans of the Royal Marines, older and really war weary soldiers, but younger, and really inexperienced blood. 

Not as a Section Lead- as a fireteam, perhaps, as my lad will be in charge of a 12 man section of Royal Marines. I did forget to mention that- it will be on a Section-Level, with NPCs in the gaps of Fireteams, but you will fill most of the roles in with human players. As the team is helicopter-bourne, it's the easiest way to fufil that sort of feeling, and give a sort of urban-assault playout.