glenmore, completely agree on your point about going upto 18. The fact is at 18, academically, there is hardly much to chose between these 2 schools. I analysed it on very many different factors including subjects the leavers pursue after 18.
Whereas at KS3 the difference might appear larger; the point was it did not seem relevant.
In reality, we are done with entrance tests until her 18 now. We maybe bit old fashioned in believing academics is dependent on the child, much more than on the school; provided the school is decent enough. Actually we could have even settled for Northwood college, to be honest. The only thing we are looking for is a decent environment, no bullying and such other soft factors. A little aura / polish, maybe (we noticed, plenty of that in St Helens). Plus there is always a chance that too much forced academics will make the child dull; which would only show up in later part of life.
She did indeed qualify for Orley Farm School in Harrow at 4+, but the location of the school is not very convenient. We cant keep on relocating as well :) so...
Overall I think both schools are quite good and in our analysis, academically equivalent. Our choice therefore was based on something else, which is individual choice and in our case which meant St. Helens.