I think HSTA is more advanced and challenging. I did HSC once, and it just didn't do much for me. Kind of too much " a little bit of this and a little bit of that" to really be effective at much of anything.
The only thing I don't like about HSTA is the way the music is handled. When the music slows down or speeds up between cardio and weight exercises, it's painfully obvious, like some kid just turning the dial on a player that can be sped up and slowed.