I'm currently Plat in both SPG and Marriott.

I prefer to stay at SPG properties especially for personal travel because they have the nicest resorts consistently in the places I want to go.

For business travel, especially in the midwest, SPG does not have a lot of choices. That is why I tend to use the Marriott properties when travelling for business, or when I want a comfortable, inexpensive room at a non-resort location for personal travel.

Another nice thing about both programs is that they offer lifetime status.