How much is each type of rewards point worth?
You also need to take into account the value of each point.
I'd put the value of an HHonors point at ~$.005 and the value of a Starpoint at ~$.04. That gives a ratio of 1 Starpoint = 8 HHonors points. In that case, any spend that gives less than 8 HHonors points/$ would be better to use Starwood AmEx.