Agree with the recommendation, but if the OP stays downtown, that's a different Marriott - simply known as Kansas City Marriott Downtown (I think). The CC Plaza is about five miles south of downtown.

Both of these Marriotts are fine - very "brand standard" properties. The downtown Marriott is walking distance to the Power & Light district - lots of bars and restaurants but little shopping. The CC Plaza Marriott is walking distance to tons of shopping, two excellent art museums, and quite a few bars and restaurants.

The Overland Park Marriott is about a half-hour south of downtown Kansas City. It's good if you have business out that way but not really useful if you want to see Kansas City.

The airport Marriott is actually quite good as airport hotels go. Certainly KC's best airport hotel and better than airport hotels in many cities. MCI is out in the middle of rural Platte County so not much in the way of nearby entertainment. You'd be having dinner at the lobby bar.

Two RI's are well-located: the aforementioned Union Hill and another one near the Plaza.

Go for the Country Club Plaza or Downtown if you have a choice...