I think the appeal is that SPG has more airline transfer partners than any other program, and the 25% automatic bonus eliminates the need to wait for a promotion.

The appeal of Chase Ultimate Rewards is attributable largely to one partner -- United Airlines. I do not foresee a circumstance where Chase would lose UA as a partner, but I do foresee UA devaluing their partner redemption chart, and when this occurs the lack of other Star Alliance options will be painfully apparent.