ODST is more of an expansion pack than an actual full game. Halo Reach is a full game.
So, which one is better? Depends on your tastes. I personally am not a big fan of the Halo Reach gameplay style (too complicated for my tastes), but lots of people like Halo Reach.
Before you can tell which one is better, understand that ODST only includes a new campaign and firefight mode. Compared to Halo Reach which has a new campaign, new modes, and a new multiplayer experience, you'd probably enjoy Reach more. I just personally enjoy the Halo 3 type of gameplay more :P