Chatsworth would be my choice - might be a good idea to be at the school gates at 'hometime' and have a chat with one or two of the parents and see if they're happy with the school (you could do this for both schools). Might also be worth phoning the Head at Chatsworth and talking it through with her and get her honest opinion as she used to be the Head at OFS.
I might be biased as I know people who sent their kids to OFS and got them into UWC as soon as their elementary school opened a couple of years ago. The problem with OFS is that an awful lot of money goes to the Directors of the school rather than into new books & equipment, so there was almost a staff revolt a few years back, quite a few teachers resigned or were sacked when they were asking too many questions. The profit-making schools tend to get away with it as they know that in most cases expat employer's are footing the bill and the parents don't ask too many questions about how the schools are being funded.
Also, check about the mix of nationalities of the kids - I think the majority is now Korean in OFS, don't know about Chatsworth. My Danish neighbour pulled their 8 year old daughter out of OFS and put her in Dover Court (her English was not good enough for her to join her siblings at UWC). Although her English was not great, she had to assist another girl who had recently arrived and whose English was worse! Her parents were very displeased and the school tried to play down the extent of how much this was going on. Also there was a teenager who stabbed a fellow pupil at OFS, it was reported in the press but then there was no follow up as to what happened afterwards.
I'm from the UK and I do know that some of the Tanglin teachers have good things to say about Chatsworth, apparently it is a very caring school. United World College and Tanglin are superb but are very difficult to get into and often have long waiting lists, but Chatsworth is also getting a very good academic and caring reputation.
