Having stayed many times at both Mill Creek CG at Mackinaw City south of the bridge and Straits State Park in St Ignace north of the bridge, both are very nice and are large in site numbers.
The real difference is Mill Creek has a playground, a pool, a sand beach, and a game room for those with smaller kids along who want the campground to babysit them for hours at a time. Lots of noise and kids running around all the time. Has about 600 campsites in several sections on 310 acres.
While Straits State Park generally appeals to older adults or having older kids along as it's a purely natural nature setting and usually is very quiet camping when compared. Has roughly 275 campsites in 3 sections on 181 acres.
Almost apples and oranges! Pick the one that suits your needs and wants.
Both CG's have great views of the huge 5 mile long suspension bridge (longest in the world for many years and still is the longest suspension bridge with only two towers between end anchorages) both day and night. Dwarfs the Golden Gate Bridge in length. The Big Mac is 26,371 feet long and the Golden Gate is only 8,980 feet long. Both are high above the water below, approx 200 feet.