The fair place to meet is Detroit, MI — the city closest to the midpoint of Buffalo and Milwaukee. From the farther side that’s about 5 hr 17 min of driving.
Recommended midpoint
Detroit, MI
From Buffalo
4 hr 31 min
215 mi to Detroit
From Milwaukee
5 hr 17 min
252 mi to Detroit
Buffalo has the shorter trip; the split is off by about 46 min. The alternatives below can even it out. Buffalo and Milwaukee are about 456 miles apart.
Buffalo and Milwaukee are about 456 miles apart by road. Split the difference and you arrive near Detroit, the city closest to the halfway point between them. That puts roughly 4 hr 31 min of driving on the Buffalo side and 5 hr 17 min on the Milwaukee side — the fairest single meeting point among the cities near the middle.
That's a half-day drive from each side, so Detroit suits an overnight or a weekend rather than a quick coffee — long enough to want a reason to stay, short enough to drive.
If Detroit doesn't have what you're after, Cleveland and Columbus are also close to the midpoint and worth a look — each keeps the drive reasonably balanced between Buffalo and Milwaukee.
Detroit is the most populous city in the U.S. state of Michigan. It is situated on the bank of the Detroit River across from the Canadian city of Windsor, Ontario. It is the 26th-most populous city in the United States and the largest U.S. Read more →
Buffalo is a city in the U.S. state of New York. It lies in Western New York on the eastern shore of Lake Erie and at the head of the Niagara River on the Canada–United States border. Read more →
Milwaukee is the most populous city in the U.S. state of Wisconsin. It is located on the western shore of Lake Michigan at the confluence of the Milwaukee, Menomonee, and Kinnickinnic Rivers. Read more →
City descriptions adapted from Wikipedia (CC BY-SA); photos via Wikimedia Commons, credited above.
Estimates use straight-line distance and typical road speeds; real drive times vary with route and traffic.