Other than the schools and important factor boils down to MYP / IB vs UK national curriculum.
MYP has just started at West Island. It is so new, a staff (non-academic) member we ran into a few weeks ago was not even aware of it. The ESF, based on our experience with Kennedy school is not good with implementing new programs. 5 years into PYP at Kennedy we were still hearing excuses like "this is new .. we're learning" and all that garbage. Perhaps that was result of the lack of leadership at Kennedy, perhaps not. Bottom line, we left Kennedy with little trust in the ESF's ability to establish basic standards across all schools.
The leadership at WIS is stronger than Kennedy, but still.. the question remains - do you want to be doing MYP or straight and standard UK national curriculum that Kellett follows. We know a few students (and parents) who are stressed about MYP and we also know parents and students who have taken it in their stride and are doing well.
Class and school sizes are another issue.
Kellett class size is 23 and they have 4 houses per year. WIS has a higher density (around 30 a class?) and more houses (6). This results in double the number of kids per year at WIS compared to Kellett.
We personally prefer the lower density and smaller year size - much more focus on the kids and over all better individualisation for the kids in my opinion.
Going into Y7 from ESF to Kellett was a smooth experience for our child -- once he got used to being given homework. First few weeks were a bit stressful as in our experience there was little or no homework in the ESF (specially Mandarin etc) and 4-6 hours homework a week at Kellett.
The staff at Kellett are very motivated and extremely professional in my experience with them and the core philosophies of the school are more in tune with what we expect from a school. Looking back at West Island I felt it was more of an academic factory.
Needless to say, the lower class density and fewer classes is reflected on the fees you pay at Kellett - if this is not an issue, I'd recommend taking a second and third look.
best of luck.