I'm Gold on UA, and unfortunately I cannot use UA MP points to upgrade coach to biz on other *A airlines. Since I want to sometimes not fly UA (i.e. if I want to upgrade ANZealand coach to biz) I figure I need points on some other *A airline. I have a bunch of AMEX points I can convert to either ANA or Singapore. Which would be the better of the two to park points in for future use, likely as upgrades on various non-UA *A airlines?

Thanks