Congratulations!
For tomorrow, you could do back, but remember that you would be working the shoulders 2 days in a row (front shoulders are worked during chest, and rear shoulders during back). I have a feeling that if your chest is feeling well worked today, your shoulders might be feeling it as well tomorrow.
Another thing to think about is that the cardio in CTX is pretty high intensity, so it might be a good idea to make sure the cardio is different from today (I don't have CTX anymore, so I don't remember what cardio goes with what weight workout). How about leaner legs tomorrow, a break the next day (to not do legs two days in a row) and Power Circuit the next day?
