The long discussion. I think we had this elsewhere recently. There are about an equal number of CPA and lawyers that claim you are "engaged in a US trade or business" as are against it. And most of the time the reasoning completely lacks any sense of understanding the law.
Now, the bigger question is where your company is effectively managed from. Could you please elaborate?
If you believe the ETBUS guys, note that paying taxes in the US does not exempt you from paying taxes again at the place of management. Also, even believing the ETBUS guys, you may be able to benefit from a treaty, exempting you from paying tax in the US.
Long discussions, no easy solutions...
For Singapore, as far as I know, they are more lenient in terms of where taxes are paid than Hong Kong. It seems the better choice to me.