I agree with Clare and dlburner. Both great for different reasons.
Ab Hits gives you more than 14 stand-alone, as-originally-produced ab routines AND several more premixes of these routines, most of which combine plank/core work and traditional ab work (which will NEVER go out of style) and running times that are more friendly to time-pressed exercisers.
Core Max gives you three stand-alone, as-originally-produced ab/core routines and three tough premixes, and the running times of these are for when you have a little more time available and a little more potential fire in the belly, as it were.
If you have to buy one at this time for financial reasons, in all honesty I'd select Ab Hits, but keep Core Max in line for your next purchase. Ab Hits does not incorporate any stability ball or med ball work, now two mainstays of any good, challenging overall core program.
A-Jock