Kilemall - There is nothing a Marine can do that a Soldier can't do (albeit with less publicity). The Army (Surface Defense Force) provides ship's troops. I dropped Marines for the same reason I dropped the Mercenary and Pirate career (Too much overlap).
While they're obviously not the same, there is a pretty strong distinction between SSD and Marines when we look at them in the RAWs.
Marine: Zero-G/1, Comms, Demolitions, Gun Combat, Gunnery, Melee Combat, Battle Dress.
Surface System Defense: Gun Combat/1, Mechanic, Gunnery, Melee Combat, Recon, Battle Dress.
The Marine is automatically a qualified spaceman.They have Comms and Demolitions, allowing hull breaches and coordinate assaults with their basic skillset, as well as retaining comms with their parent ship for coordination. A focus on Int/End in their qualification and survival rolls suggests your average marine is resilient and resourceful, both vital in space combat.
The Surface System Defender/Soldier is different. They start with Gun Combat/1, meaning your average soldier beats your average Marine in a gunfight. They start with Recon/Mechanics, meaning they can both have the initiative in combat and keep their gear running over extended fights. Their qualification/survival are End/Edu, so on average your SSD will have more advanced education skills than a Marine.
Also notable here is that the Marine qualifies on Int, survives on End, commissions on Edu, and advances on Soc. Soldier quals and commissions on End and survives/advances on Edu. That means it's WAY easier to stay a Soldier than it is a Marine.
The point here is that your initial premise 'There's nothing a Marine can't do that a Soldier can't do' is wrong, since the Soldier career will never give you any relevant space skills. You'll probably get more skills and promotions from being in Surface Defense, but the Marines are the only ones who get Zero-G.
Unless you've tweaked the tables or replaced one of the servicers with Zero-G (my usual go-to). Then all bets are off.
