We're at ICMS and are quite happy - although in some ways, I wish we'd chosen ESF just so that getting into primary would be easier.
Here's my pros for each place:
ICMS - smaller class size, teachers work virtually one on one with students for a significant amount of time, location is better for me, I think just for kindy, I like the school better. My daughter is really thriving there.
ESF - priority to get into ESF primary is the biggest factor. Also I think it's a bit cheaper. I think probably the teachers are paid more and may be more highly qualified but not 100% sure on this one. One issue with ICMS is that they do have a significant amount of staff turnover.
That said, I've been very happy with ICMS and am hoping that as my husband is ESF alumni, that priority is high enough to get us into primary (it should - it means that we have priority over people with NO connection to ESF... but it is the lowest form of priority, below siblings, ESF kindy students, and children of staff). If I didn't have that though, I would probably be regretting not sending her to ESF, purely due to the primary situation - we do hope to go to Quarry Bay primary, most probably - but will probably be applying to at least one other school just in case we don't get in.