Hi Preghead - you must live close to us, as we are also in the catchment for RAAS 
(We made an application there as an 'insurance' option - just in case anything went wrong, or we had a sudden family financial crisis or something!)
I think RGS is slightly more academic, although the I'm not sure the FT charts reflect this? There seem to be more A*s at Caterham than at RGS?
We've been talking to lots of friends with kids at RGS, and also those who know people at Caterham etc, and I think we are going to head to RGS.
I just don't fancy commute to Caterham. Even if DS got the bus (which he could do) there will always be days when he's going in late, or misses it, or has to stay late etc etc. DH & I both work, and it will be a nightmare to have to be standby taxis!
One of my very good friends with children at RGS says she thinks RGS recruits more academic kids in the first place, and then they continue through at high academic levels. She thinks Caterham 'adds more value' to get bright children to attain higher grades, but she also thinks the pressure is greater at Caterham? Another friend said she thnks all the scholars are in the same class at Caterham which creates a lot of pressure, and a bit of a 'hot house', whereas at RGS they are spread across the classes (and the classes mix much more anyway).
The scholarships are not worth much in the grand scheme of things. The RGS one is worth £1000 per year (not term) and hasn't changed for many years, so is diminishing every year in real terms.
DS's Caterham scholarship is 20%, which is rather tempting (and works out about 2.5 x that of RGS) but the cost of the bus would wipe out a big chunk of that!
I think people who are expats, or relocated by their company often get school fees paid as part of their allowances
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RAAS has only just opened a 6th form, so no results for it yet. In the past kids have gone to Reigate College which features about 1,200th in the tables!
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The GCSE results from RAAS are not bad (National average?) but are improving rapidly. Definitely worth keeping an eye on.