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in HALO 3 you are a Spartan and in HALO 3 ODST you are a Orbital Drop Strike Trooper (ODST)
I would say Halo 3 ODST
Halo 3 ODST is not for PC but Halo 1 and maybe Halo 2-3 are
Halo 3: ODST is the newest of the two, but not the newest Halo game.
halo 3 odst, unlike halo 3, has no matchmaking, sorry