Typically I prefer FFI as TPS seems a little too limited in its service for me. And it's also 5x points instead of 10x on paid stays. OTOH, TPS rooms all have kitchenettes with full size refrigerators, sink, microwave, and range top. For people with certain dietary needs/preferences that can be a big plus.

If none of these standard feature differences mean a lot to you, try browsing photos of the two specific properties to see if one appeals to you more than the other on something you care about. Look on Marriott's site as well as sites with user generated content. I do this, for example to see if one hotel has a more attractive/functional pool area than the other, or a better exercise room, or more modern/pleasing decor in the guest rooms.