Re:KC: I-70 from STL or I-35 from DSM?
This is a "discussion" Mrs. Roadhouse and I have been having for close to 35 years of regular travel between Chicago and KC. I-80 to Des Moines, then south on I-35 is in fact the shorter route. It is also bleaker, more desolate, and more likely to put you to sleep than the I-55/I-70 route.
Don't get me wrong, I-70 isn't exactly a treasure trove of fascinating roadside attractions and hidden dining gems, although, they exist to some degree. But it does give you more more, and closer sources for help in case of an emergency, and better options for getting off the road for breaks in well lit, well populated areas.
You've got the option of running through St. Louis (nothing beats a dead-of-winter stop for frozen custard at Ted Drewes), or if you're trying to make time, bypassing the city on I-270, then hooking back up with I-70 a few miles west.
All that being said, on our last trip, back in December, we opted to take I-72 west out of Springfield (IL), which turns into state route 36 in Missouri. 36 runs into I-35 which we then took into KC. This was strictly a time decision. The I-72/36 route rivals the Iowa path for sheer mind numbing dullness. If we'd gotten an earlier start, and if we weren't staying at an apartment that was so far north of town, i would have definitely taken the I-70 option.
Have a great trip!