UMSC is private. It was set up to encourage higher-ranking, important UMMC doctors to stay with UMMC by giving them the opportunity to earn extra income in a private setting, instead of leaving UMMC to earn big bucks with other private hospitals, so UMSC does use some UMMC facilities. For example, the UMSC maternity ward is in the same building as UMMC's maternity ward. You get your own room. OK la. If you want better hotel-style treatment, better go with other private hospitals, but if you want top-notch doctors who really care about people (that's why they still work at the government teaching hospitals even though they could earn better elsewhere), then that's why you should go to UMSC or UKMSC.
They are different, so which is better depends on your preferences.

Prof Siti is like a strict mother/teacher. I went with Prof Siti for my firstborn. I got gestational diabetes and she insisted that I report my blood sugar to her three times a week, keep a diet diary and show her.  I had absolute trust that what she did was good for me and baby. She won't suggest any tests/treatments that is not completely necessary, not even painkillers. You have to demand for painkillers or other tests/treatment if you want them because she won't suggest them. Under her, I had ultrasound only twice the entire pregnancy. She takes her time with you, so the wait can be very long, because she spends a while with each patient.

I went with Prof Tan for my second baby, born in May 2015, because Prof Siti was on sabbatical in early 2015. I'm not sure that she's back from her UMSC sabbatical yet, though I heard she's back to work at UMMC.

PC Tan is an ultrasound specialist, so every visit he will use the ultrasound machine. He's more jolly in attitude than Prof Siti, and explains things patiently and very well. He likes to give suggestions to you and it's up to you to decide yes or no, but he's very efficient at checkup and you're out the door quite promptly.