SW Mich. to Saco,ME. in June I-80 or I-90?

I-90 would not be my first choice, but perhaps that's partly because I've traversed the NY Thruway too many times already. Be prepared to personally put a little dent in the NY highway budget if you do take that route. (I'm not sure it's actually extra expensive per mile, but it's likewise not the least expensive, and there are a lot of miles from end to end.)

I would probably go I-90 to I-86 to I-88 and then perhaps route 2 to through Massachusetts to I-495. Some here detest I-88, but I've never found it to be a particularly bad road myself.