Having summered in the U.P. several times, and visited lakeshore Minnesota, I would say your chances of cool weather are about the same both places if you are on the Lake Superior shore, but most of the rest of Minnesota might as well be in the Dakotas or Iowa, hot continental summer climate.
That being said, one late June day when I was in Marquette it set a record as the hottest place in the U.S. that day, and the next day it was the coldest. The storm in between was really interesting and I was happy not to be camping that night.
No guarantees.