Not sure what the rule is on stopovers - isn't it 24 hours or less?
As for SPG / BMI - Amex Membership Rewards goes 1:1 into BMI but 1:2 into SPG. Using Amex as a base currency, 1 BMI mile would be worth 0.5 SPG points.
On the other hand, SPG points can be transferred to most mileage schemes (but not BMI) at a 1:1 ratio.