First off, I'll say that we live in the suburbs, North of Kansas City, and don't think I've traveled either route the entire length, just protons of both. If we are going either East or West, we'll get on I-435 to I-70, but that will be way off route for you.
I'd say that either I-670 or 470 would work, but 670 would probably be shorter. That time of night there "shouldn't" be that much traffic no matter which way you go. Unless you catch traffic from from the KC Royals game. Royal's Stadium is on the East side of KC, in the I-435 (just East of it) . They do have a game starting at 6:15 pm Saturday. If you come in earlier than 7:00 you may get caught in a MESS. By 7:00 though it "should" have settled down. Also, if you choose to continue on I-70 again the traffic shouldn't be too bad, but once you get into the downtown area you really need to watch the signs, or listen to your GPS. It can get pretty confusing quick!
Should you decide you want to spend the night there is a decent Good Sam's park in Grain Valley, East of KC, here is a
LINK. We just drove through there a few days ago, checking it out in case someone was interested. It's a little tight, but for an overnight it wouldn't be bad. It's just off I-70, with fairly easy access on & off. Also, there we have construction going on in different areas, here is a
LINK for MDOT, with construction zones.
Hope this helps, I'll PM you my phone # in case you have questions, or anything.
Roy