They are to diferent except in the game engine, odst its not aprequel or a sequel, it has diferent gameplay, same timeline but not campaign or story, and diferent character, also it feels like the previous halo 1 and feels like a total diferent game, thats my opinion.
1 REASON FORGE WHO DOESN'T LIKE FORGE DOES ODST GOT FORGE I DON'T THINK SOO
dude... ODST DOES have forge, it actually ahs forge on maps taht you can't download for a conventional Halo 3 disc.
Anyway, I liked the both a lot, but Halo 3 better, cuz of the immense multiplayer. Wave-based multiplayers only go so far. It is safe to say that the campaigns, while differ in length, were equal in AWESOME quality. Oh yea, and Fire fight makes mincemeat out of Horde (Gears of War), destroys Survival (L4D), and makes Nazi Zombies crawl into a corner and whimper! (CoD5)