My question is, why not attend law school in Singapore if you plan to live there? That would make the most sense.
Other than that, I think you'd be better off with a UK law degree regardless of whether or not the program is shorter, since the Singaporean system has more in common with the UK than with the US. Additionally, although I have no idea what your numeric qualifications are, getting accepted to Harvard, etc is no small feat.
I doubt if the time to complete the degree and begin practicing is all that different between the US and UK schools, maybe one year at the most. Even if you finish the UK law degree in two years you'll still have to prepare for the Singapore bar exams (I presume), and wait for results.
If you do in fact get accepted to someplace like Harvard, however, the degree has much more international cache than the vast majority of UK universities, save for Oxford/Cambridge. I've travelled all over the world, and elite US universities such as those listed are held in very high regard just about everywhere. That could be a huge advantage.