I'm going to be driving through Michigan into Canada next week. My point of origin and destination are far enough to the West and East respectively that the net mileage difference between using the Blue Water Bridge in Port Huron, and the Ambassador Bridge in Detroit is insignificant (7 miles).
Similarly, the difference in tolls is insignificant. (Two dollars)
Which leaves me wondering whether there is any difference between the two in terms of traffic patterns or customs staffing, wherein one crossing would be faster than the other. All things being equal, I'd intuit that the Port Huron crossing is going to be faster, if only because it avoids crossing through Detroit commuter traffic, but I won't be traveling at rush hour, so I'm not particularly concerned with this factor, if there is some compelling reason to go the southern route.