ODST does'nt have the AR.
in HALO 3 you are a Spartan and in HALO 3 ODST you are a Orbital Drop Strike Trooper (ODST)
In halo reach there is some small moderations. The Rifle is slighty more accurate and has a new visual upgrade. But the assault rifle in Halo 3 is much better becuase it has more ammo (the rifle in reach only has 32 and the rifle in halo 3 has 60) and is the original version.
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
The 3 round burst gun from halo3.
It is hanging on the power lines its a bent sniper rifle.
Halo 3 ODST will be released on September 22
Yes you can get the mongoose on Halo ODST
Yes and no. You do not need Halo 3 to play ODST. And you do not need to have played Halo 3 to play ODST. It is an expansion of the Halo storyline. I happens concurrently. No its not this is not an expansion of the story this is just another part of the story.... so basically whats happening in the game halo 3 is happening at the same time as whats happening in halo 3 odst.... They are just two different battle fields. ODST are not Spartans either they are like elite marines Halo 3 odst is between halo 2 and halo 3.