Since you're considering Kennedy, you're obviously looking at Primary, not Secondary, so the West Island comments are not (yet) applicable. My daughter goes to ESF at QBS. Personally I cannot justify almost 3 times higher fees. I honestly don't think my child will get 3 x better education for that money (besides the fact that I have 3 kids ...).
I also don't hear the greatest things about HKIS lower primary (friend who pulled 3 kids out, other friend who regrets her choice, etc.). It's a little too free for me, I prefer more structure and academic approach (at a slow rate combined with play), perhaps a more traditional style of education. However, I heard secondary is very very good. Facilities are great, but at this point, we're not willing to pay that much extra for that, it's quite simple, we pay from our own pocket.
Also, I am not at all religious, so that's also a concern to me (my friend's daughter came home that God created Earth ... fine, but not everybody shares that view). Overall, I've been happy with ESF, yes, classes are quite big, it's not super fancy in terms of facilities but it's well structured and organized, at least at my daughter's school. She progressed quite impressively, integrates very well (she's Caucasian, blond and the school looks / is very Asian but not necessarily Chinese). The level is very good and it's quite down to earth. It's good quality education for a good price (for HK international school standards that is). Even though my daughter is American, and we hope to return one day, I don't regret our choice at all. On the other hand, I know plenty of people who are quite happy with HKIS, but most are American, religion may be less of an issue, they'll most likely return to the US at some point and quite a lot have education paid for by the company so the money is not an issue. Different things to consider.
Good luck with your decision.