I spend a good deal of time in St Louis, mostly the western and northwestern suburbs.
To me, St Louis feels "bigger" than KC. Not sure how much the actual population differs between the two.
It definitely feels older as well. Some of the suburbs mentioned already, Ladue, Chesterfield, Clayton, are all very nice places. If I were ever to relocate to the area, those would be my choice.
I will say that St Louis has traffic like a city twice its size. Proportionally it's pretty bad. (DC/Baltimore is the worst, though)