I've traveled both many times and IMO it's pretty much a tossup. To me it mostly depends on which has had the latest maintenance done. Hopefully you'll get some responses from people that have driven them recently enough to give first hand data on road conditions.
If it were me I'd probably take 74/72 to 36 Hiway across Missouri to I-29. Then decide whether to do I-70 or I-80. If you go across on 36 Hiway you'll avoid St. Louis which is really horrible. Also 36 Hiway is 4 lane almost all the way across the state now.