I agree that the Stock Yards is more interesting, but with the location, distance from Dallas, and all of the road construction along I35W, unless you know the area it is a little tricky to get to these days.
From downtown Dallas the easiest right now is probably I30 to one of the north south streets that will take you through downtown Fort Worth to the Main Street bridge behind the courthouse, then straight down Main Street to the Stock Yards. I do not know what the westbound I30 signs say about exiting to downtown Fort Worth coming from Dallas. If you can navigate to Commerce that would be the most straight forward way through downtown to the courthouse. You could come to Fort Worth early and stop by Sundance Square, which you would drive by, for dinner. This route will also take you right past my favorite Mexican food restaurant, El Rancho Grande, which is on the northeast corner of Main and Central Avenue. In the Stock Yards is the Lonesome Dove Western Bistro.