Well, the good thing about UWC is that at the end of Grade 10 the pupils take the IGCSE (International General Certificate of Secondary Education) exams. This would be a great 'practice run' in taking exams for your eldest no matter what educational system your daughter will return to, and if your daughter in years to come wishes to work in say, the UK or in countries like Hong Kong, Malaysia, Singapore, Brunei, India (ok - ex-British colonies!) then as the IGCSE's are basically the English modern version of the "O" Level qualifications (which all Singaporean local pupils take) it might be nice for her to have taken the exams.
My daughter completed Grade 10 and took the exams this Summer and had excellent results - a tribute to the fantastic education she received at UWC. She had American teachers for art and math. Have you seen their website? They have fantastic educational trips, there is one coming up in India, but they also take pupils to places like New Zealand, Oz, Europe etc. You can even go on a family trek to the Himalayas at Christmastime, an Aussie family I know did this trip and said it was wonderful and the memories will last a lifetime.
Also the vacations I think are brillian for expat families with kids at UWC - they get long breaks at Christmas, a week in October, Feb, 2 weeks min at Easter and about 7-8 weeks in the Summer. It gives you lots of opportunities to take vacations around the region, to places like Phuket, Bali, Hong Kong etc. etc. I know from SAS families that they did not get as many school vacations and not so many opportunities to visit other Asian destinations. I know that both schools are very highly regarded, so whichever one you choose you will have no worries.
http://www.uwcsea.edu.sg