Don't go through the city. Beyon that, it really depends on your priorities.
If you want to avoid tolls, going west to I39 would cut some out, but it also adds 40 miles to the trip - which means both time and gas. Taking 294 up to 88 and crossing west there would cut some of the tolls, but would reduce the added miles from 40 to 15. I-90 is obviously the straightest shot you have, but would also be the most costly in regard to tolls.
Traffic is the bigger priority for us. The tri-state isn't too horrible if you avoid rush hour. We try to steer clear of it from 7-9:30a and 4-6:30p on weekdays. If I stear clear of those hours, I don't typically have too much trouble. For whatever reason, Sunday afternoons can be particular bad on 80/294 near the IN/IL state line, but that is often worse eastbound than it is westbound (althought either can be an issue).
All in all, I have done the drive from SE Wisconsin to SE Michigan more than a dozen times in the last 6 months (and many more over the years), and avoiding rush hour and Sunday afternoons generally keeps me out of too much trouble.