I like and use them both. The abs DVD is compilations from many of Cathe's older workouts that had ab sections. I like this DVD because there is a lot of variation and I have some favorites on there. (There are several that use planks, which I like.) You can do all kinds of variations and as many as you want. Core max on the other hand, incorporates the medicine ball and stability ball so it's completely different ab work then what's on the other DVD. There are three different workouts each about 20 minutes. The first workout doesn't use any equipment.
Do you have equipment? Do you like traditional ab work? What are you looking for? Honestly I use them both so I find it difficult to pick one over the other. If you don't have a stability ball or medicine ball and don't plan on purchasing either, then I'd get the Abs workout. Do you own other Cathe workouts that have ab work like ME or Muscle Max or any of the others?
Get both!
Jo