I've owned BMWs with both sport and nonsport seats and they are both comfortable. The sport seats grip the driver better for "sporting" driving, and the non-sport fit some folk better for long distance interstate travel. Try both & then make your choice.
Also, many of us with back problems really appreciate the lumbar adjustment, but unfortunately, it can only be ordered as part of the Premium Package. If you will be getting most of the other PP stuff (power seats, etc.), you might consider it.