What are you planning on doing in Dallas? I would rent a car for sure (I live here) and you can't get around without one.
The Omni is nice, but downtown is not a place I like to be ever, and definetley not at night. The Westin in the Galleria will put you closer to Addison, which has a lot of restaurants in a small area, but again, you'd need to drive there.
Rent a car and save yourself A LOT of time and cab fare.