Hi,
I think there are several things I would consider if I were in your shoes.
Firstly, it is the presence of a HUGE amount of scholars in DSTA. Given that these people are much more enthusiastic about defence science than you are, and that most of them are headed to CMU and UIUC (around the same league as UMich), it doesn't give you a significant advantage to be a scholar. Having to fight with 50 overseas scholars each year in such a small organization rather than the entire civil service (for ~50 PSC scholars), the path of a DSTA scholar will definitely be infused with more cut-throat competition. With that in mind, you also have to consider if you can get the jobs you want in DSTA when you graduate. Limited positions in certain areas, compounded by the influx of scholars each year is bound to produce disappointment as many will not get their dream jobs even within DSTA.
Secondly, the alternatives you have in Singapore isn't that bad either. At SMU, they give out scholarships which cover your entire tuition fees, laptop allowances, pay for an overseas trip and give you an annual allowance of $5,000. At NTU, the Nanyang Scholarship gives similar benefits. However, I'm not too sure about NUS. With the choice of a free education in Singapore, and the freedom to pursue whatever you want upon graduation is something that you can also consider.
The points I've mentioned above are just food for thought. That said, I'll be going to UMich this fall for engineering as well. We can communicate if you have decided on going!