DTW has a beautiful terminal many shops very new. You will be able to kill time here easily. MSP has lots of stores in it. Both have tons of sit down restaurants so thats always a nice way to kill hours. You might want to consider a day pass to the skyclub or buy one on ebay i think if you buy one you can use it in both DTW and MSP might be worth it with drinks, snacks, internet, comfortable areas, nice restrooms if your layovers are that long

Go skiing in SLC you will reduce the number of layovers and travel time after the airport