I have only dove on the Black Manta, which is the newer sister ship to the White Manta. I have been on it 4 times and quite enjoy the trip. The diving is enjoyable, if not world class. I have been both on the Tioman/Aur & Acasta rock/wrecks itineraries. Although others have different opinions I enjoyed the Tioman trips more. The reefs around Acasta rock and other sites on that itinerary are all heavily damaged from dynamite fishing.
The nice thing about these trips is most of them leave on Friday afternoon and return Sunday evening, so it's an easy chance to get in some decent diving without having to take time off work. If you are in Singapore and have only a limited amount of time, it is definitely worth the trip IMHO.
September/October is getting to be the end of the dive season here, so it is possible that the weather may start to turn worse. I wouldn't let that stop you from taking the trip, but it is something to keep in mind. The Black Manta is also a slightly bigger boat than the White Manta and is a bit more stable in the water.