There is no difference in the watch itself, just the movement. Some like the A7750, some like the A23J (ST2555). It's a closed caseback so the more accurate look of the A23J is moot, but it's 100$ cheaper. So it's really your choice to make. Repair wise, both will necessitate removal of the decorative plates to make it look like a P9000 and then re-attaching them on the new movement (if you prefer). Reliability wise, the A7750 has a longer track record with reps, but all my ST2555 movement based reps have held up so far.