I beg to differ. There are definite differences between Michigan and Michigan State in the composition of their in-state undergraduate student body, going to more than GPAs and test scores. Fortunately, both schools provide us with breakdowns of the county residency of in-state undergrads. Not surprisingly, both draw the bulk of their in-state students from the 3-county (Wayne, Oakland, Macomb) Detroit metro area—Michigan at 52.3% with MSU not far behind at 48.2% (all figures for fall 2014). But the composition of those metro-area students differs. Michigan draws 30.4% of its in-state undergrads from Oakland County (northwest suburbs), the most affluent county in the state (median family income $84,783 per U.S. census data, nearly $25K above the statewide median). MSU draws 25.4% of its in-state students from Oakland County, a healthy fraction but a decidedly smaller share. UM also draws a higher percentage from Wayne County (Detroit, the Grosse Pointes, western and downriver suburbs), 16.7% v. 14.6% for MSU, but MSU draws more from Macomb County (8.2% v. 5.2%), generally a more blue-collar suburban area than neighboring Oakland (Macomb median family income $67,423).
Perhaps the most striking difference is that UM draws a hefty 13.1% of its in-state students from affluent Washtenaw County (Ann Arbor, median family income $82,184), so that fully 65.4% --roughly two-thirds–of its in-state students come from just 4 contiguous southeast Michigan counties, including 2 of the highest-income counties in the state (Oakland and Washtenaw). (The only other county anywhere close in family income is Livingston which tilts MSU but is much smaller). In contrast, MSU draws just 51.9% from those 4 southeast Michigan counties. In part this is just home-field advantage; you’d expect more Washtenaw County residents to attend UM. MSU has a similar home-field advantage in its home county, Ingham (7.4% v. 2.5% for UM) and in surrounding counties like Clinton (2.1% MSU, 0.4% UM), Eaton (1.9% MSU, 0.5% UM), Ionia (0.5% MSU, 0.2% UM), Livingston (3.6% MSU, 1.9% UM), and Jackson (2.4% MSU, 0.9% UM). But MSU’s home turf is notably less affluent than Washtenaw County as measured by median family income (e.g., Ingham $61,680; Clinton $69,611; Eaton $66,788; Jackson $56,314; the outlier is Livingston at $82,637, but again that smallish county contributes just 3.6% of MSU’s in-state students).
More broadly, you could draw a wide arc across the state, beginning in St. Clair (Port Huron) and Macomb Counties in the east, extending west through Lapeer, Genesee (Flint), Saginaw, Bay (Bay City), Midland, Ingham (Lansing), Kent (Grand Rapids), Muskegon, and back down through Calhoun (Battle Creek) and Jackson Counties, and MSU draws a larger percentage of its in-state undergrads from each of those places—including most of the state’s secondary cities after Detroit–than does UM. Median family incomes are generally lower in these places than in UM’s southeast Michigan stronghold (e.g., Kent $61,097; Genesee $54,072; St. Clair $59,969; Bay $53,824; Calhoun $52,533; Saginaw $53,171). UM does slightly better in parts of southwest Michigan (Kalamazoo, Berrien, Allegan, and Ottawa Counties) and in the Traverse City area, though the numbers are small (2.3% Kalamazoo, 2.1% Ottawa, 1.3% Berrien, 0.5% Allegan, 1.3% Grand Traverse).
Without question there’s overlap in the in-state student bodies of the two schools, but there are also differences. There’s no question that Michigan’s in-state students skew more affluent and more southeast Michigan, especially Oakland-Wayne-Washtenaw Counties. Consistent with that, a few years ago the Michigan Daily published a list of UM’s top 20 feeder high schools; 17 of the 20 were in Oakland and Washtenaw Counties, 2 were in Wayne County (Grosse Pointe South and Detroit Renaissance, a magnet school in the Detroit public school system), and only one was elsewhere in the state—Okemos High, in an affluent suburb of Lansing.