I'm about to buy 2 economy JFK to HK tickets for US$3500 total (ouch!).

Would I get better value from using the CX Amex (double points because of foreign currency) or from the Starwood points from my US Starwood Amex?

I think I also need to allow for the US$:HK$ fx rate that CX HK uses.

Thanks (I guess this has been asked in parts around this site, but I can't find this direct comparison)