I-75 is known for being the more congested route. It tends to get much more through-traffic since it is a major north-south interstate (including much more truck traffic).
I-71 can get congested as well and will tend to back up starting around Dana Ave exit or Smith/Edwards Rd exit (exit #6) when traveling northbound from downtown Cincinnati at rush hour. It gets slow many days all the way north off and on from exit 6 to the I-275 beltway, but is not as bad as I-75 will get. I-75 can be a very slow drive all the way up from downtown to the beltway and beyond. Coming into downtown is usually not a problem on I-71 and traffic moves well in most areas in the morning (at least to my knowledge) except where the bridges come into downtown traveling from northern Kentucky neighborhoods.
Check out the local TV stations during rush hour on their web sites and you can see traffic maps showing the slow areas. That is probably the best way to tell if the route you are considering tends to be slow or not.
