Hi. I graduated from ICA, which prepared me well for college and stress in the workplace. My daughter is in Miriam Grade School (Gr. 1), mostly because of the distance from our house to ICA. Had ICA been nearer, I would have considered ICA too. What I appreciated at ICA was that it was a place which developed my spiritual side--but then this might be more due to personal disposition than the school. Nevertheless, there was a lot in the schoolyear and activities to feed it.
Value for money, ICA. Facilities are newer, and the difference between ICA and MCGS is substantial (for me at least).
Academically:
The kids at ICA are very smart and self-driven. It is true that the school administration (two nuns) are very traditional, but students (and faculty) know their way around the system and are able to initiate many things (not without some grief, but still).
Also, the two nuns have hired Queena Lee-Chua (one of its best known alumna) to help revamp the curriculum to make it more 'international', likely more progressive than traditional. She holds office at ICA 3x a week.
There is a lot of homework, and although kids in my generation coped by ourselves, it seems like a lot more ICAns go to tutorial centers these days (although mostly because of Chinese, some now because of Singapore Math). That being said, the school culture fosters self-driven children, so kids teach other kids, and people try to cope by themselves as much as they can.
It's an environment with very smart and very driven kids (peers expect it of each other), and so for laid back kids, it might not be the best fit. In addition to school (7.30-3), girls have a lot of extracurricular activities. I've observed groups (I do consultancy work for ICA) who eat lunch at 10.30 (recess) in order to practice for school activities or have meetings at lunch time. The pressure was one of the reasons (apart from the distance) I decided not to enroll my daughter at ICA.
Having enrolled her at Miriam, I know now that, although the pressure to excel exerted by peers is substantially less, there IS a lot of homework, which puts a lot of pressure on parents (I assume here that kids are happy go lucky and would be happy to go to school without doing HW), if not the kids. One of the music teachers told me, though, na sinisikmura ang anak niya dahil sa academic pressure. Again, this might be due to disposition but still, there is some sort of pressure exerted.
There ARE a lot of kids, so you can't reasonably expect personalized instruction or attention in Miriam (as is the case at ICA). So if your kid is the type that requires personalized attention, it might be best to find a more progressive learning environment.
Also, if you don't have time, as a parent, to keep track of your child's emotional concerns (yes, there is a lot of bullying going on, although having pushed hard, the school helped me in my daughter's case and addressed it pretty well) some issues might arise, given the huge size of the school population. I was bullied too at ICA at around Grade 3-5, and I hear about cases re: bullying at Grades 1 and 2. The bullying wasn't addressed mainly because I didn't tell anyone; I felt the guidance counselor liked the bullies more than me.
Miriam hasn't adopted Singapore Math yet (as ICA has), and I'm not too fond of the traditional way of teaching Math. ICA does well on the TIMMS and, if I remember correctly the MCGS principal told me that Miriam isn't doing as well, and is currently working on this issue.
One big difference is faculty morale. They seem much happier, and less scared (of parents, administrators) at Miriam. Seems like there's less turnover too. Happier teachers mean that they are possibly more likeable and likely better teachers. Although a friend of mine who worked at ICA said that teacher training provided by ICA is the best one she's received (she didn't stay though).
School spirit is strong at Miriam; it used to be not as strong at ICA but I think things are changing. ICA's a younger school, after all.
I would place my daughter in a more progressive environment if I found such a school nearer our place, which fits her disposition. (For ex. Raya School.) But she's more or less happy at Miriam. (Maybe she's more happy than me, because of the HW situation. I get depressed when exam time comes along. Haha.)
Hope useful.