I flew YYC-FRA-YYC on AC's 787-900 in J (seats 5K, 2K) recently, and connected to LH flights to RUH seated in J, as well (seat 3G on a 340-300 and 330-300), and I would recommend that 789 hands-down. The service was very good, the food decent, and seat layout stellar (1-2-1), and the seats quite comfortable. I love the 789 for how quiet it is and the lower cabin altitude, as well.

Some of the LH C still has the /\ shape with limited dividers, which can mean playing footsie with your neighbor (it did for me on the 340). Ask if there are other areas you'd find of greater concern.