There are now three Marriotts in the Cincinnati area. One is up near the Kings Island amusement park, off I-71, about 20 miles from downtown. Two are actually in Kentucky, one directly across the downtown Cininnati on the river, and one at the airport. Without knowing more about your meeting (size, purpose, budget, etc etc) it's hard to say which would be best in your case.
We've stayed at the one on I-71; it's nothing special, but as I recall there is a fair amount of meeting space. Both the others are quite new - the one at the airport is about a year old or less. The one on the river is a 2-3 years old now, and seems very popular.
For convenience for attendees flying in, you couldn't beat the airport of course. If your attendees will want access to restaurants, nightlife, etc, the one in Covington is the obvious choice.