Halo 3 online multiplayer is WAY better then Halo odst online multiplayer
Here are the reasons:
1.Halo 3 has lots of game modes online unlike Halo Odst ex.Team Slayer, Lone Wolves, Custom Games, Social Big team, Rumble pit,co-op campaign and more
2.Halo odst only has 2 multiplayer options which are Co-op Campaign and Firefight
3.In Halo odst there is no matchmaking in Co-op Campaign or firefight...you Have to invite your friends to play with you
4.Halo 3 has matchmaking unlike Halo Odst
i hope this information helped!
P.S. when you buy Halo ODST it automatically comes with a disc containing all the Halo 3 multiplayer maps a 2 in 1 bundle