STS. It has more variety in training with quite a few segments (six, I think) using yoga, med ball, stability ball, slide discs, Pilates, and no equipment. It is tough stuff!
I am going to be the dissenter here. Although I LOVE both Ab Cicuits and Core Max, Core Max 2 and 3 are my very favorites, and the ones I keep returning to again and again and again! (and they whipped my post c-section abs into fabulous shape like nothing else was able to do.......of course, this was before Ab circuits came out)
I'd go for STS. I like them both, but CM is more traditional and STS has more variety. I'm not a fan of ab work anyway, but I really like the STS abs. I think they're both roughly the same price, with STS you get 6, with CM you get 3.
I vote for STS AB Circuits. I love that all the moves are slow and controlled. Not a million frantic crunches like a lot of ab workouts! Love Love the med ball one.
I agree with the others that Ab Circuits was the better choice. It has more variety, and particularly if you have some back issues, you'll have more flexibility with that choice. Enjoy!