I think a lot of it depends on where in northern WI and what time you are coming through. The I39-24 route adds 100-125 miles to the trip (depending on which part of WI you are coming from). That's 2 extra hours of driving and $30+ additional in gas (for us, the gas cost would be closer to $45). I could deal with some traffic and cover the cost of a lot of tolls for that. Even going down to I80 adds 60 miles as opposed to coming straight in from Rockford (if you are that far west to begin with).
If you absolutely have to come through between 7-10a or 4-7p, then it might be worth the the extra distance to save the frustration, but it's doubtful. Generally, coming from the Milwaukee area, I just make sure to avoid those times, use the tri-state (294), and don't have many difficulties. It would really take a major accident in the midst of rush hour to have delays as extensive as those 2 hours.
Unless you are averse to heavy traffic, I would suggest you do the 39-80 route or just try to time it for midday and come right down 90 to 294 and then around to 80 (the latter would be my option).