Northwest corner of town is where all of the newest homes & apartments are going. I would say it is generally considered the "better" part of town, but maybe "newer" is more accurate. Outside of loop 250 going due north and due west there are some nice areas going up also. You can go rural in almost any direction depending on what you are looking for. There are good areas inside the loop, but I am less familiar with those.
Downtown is trying to get better with the new energy tower and word of condos being built there.
The term "south of the tracks" applies to both Midland and Odessa, but some of the best local restaurants can be found there.