Its mainly the human aspect that is different, i think you already know that.
The teachers interract very differently with the pupils. In the Simgaporean system they are more likely to point out a pupil's shortcomings whilst in an International school its a more positive parenting attitude.
The system is part of a wider extremely condescending paternalistic ethos, where "esprit de corps" networking, allegiance and service are pivotal values. No place for dissent, not even a bit of creativity, no slacking, no pacing.
You'd have to feel at ease with that. Certainly if you are a PR/part Singaporean family you should consider."Esprit de corps" is very important when it comes to finding a job one day.
A good friend (PR) is a surgeon with husband lawyer. They removed their child from tanglin (where he was laready doing extremely well) and moved to a prominent local institution as he will grow to be part of a circle that will serve his future best. And as he will go to national service, he will have the camaraderie of fellow pupils which will no doubt help on the social ladder.
Another local friend has their kids at TTS and shiiped to oarding school overseas at 12 as they just can not handle the total lack of empathy and "missionary" ethos.
They are not 100% ethnically chinese so that also says something as to how within even thosevpublic schools there is a social stratification that you'd need to be at ease with.
Anyway, lets no go into that territory, but hey ho, sometimes things need to be said.
So really what am saying, without any judgement at all, is that you know best what will suit your child and your family dynamic. I know a lot of happy parents/kids at local schools. They are all in it for the long run.
I only have good things to say about TTS from experience. They actually do a pretty good job at "pushing" kids with a potential and generally keep a busy reading and homework schedule from an early age.
They are however quite opaque with the parents as to the ability grouping but its easy to figure out if you volunteer as a parent reader for instance. So its very much to the parents to keep on top of it.
UWC and TTS are always on top of listings for a good reason, they are academically very sound yet nurture a whole person.
They provide a happier environement and let us pushy parents do the dirty job 
i feel that schools should help identify talent, abilty and areas of interest. Its my job as a parent to keep the whip on nudge my child in the right direction to fulfil the potential.