If I were you, I must choose St Mary. My supervisor's 2 kids also studied in St Mary before. Her eldest kid's finished Form 7 and her English is very good and conduct is very good. Her youngest was studying in secondary school but it is not St Mary as her standard didn't meet St Mary's requirement. Once she went to another secondary school. She is always the first one in the form although it is brand 2 school.
One more thing, her parent shared with me is quite important. If your kids study in the same primary and secondary school. The peer group of your kids will be much better. At least, you know most of them very well since they study in the primary school.