Mapquest is right. Take 35 to Des Moines, then hop on 80 into Chicago. Taking 70 to St. Louis and then 55 on up is a nightmare. Every inch of that road makes you want to tear out your soul.
Having lived right off of I-70 in Missouri for the vast majority of my life...yes, stay off of that highway at all cost. Going up 35 and on 80 is far less traffic, roads are far better, and scenery overall is more enjoyable too.