Depends on a number of factors. A wetsuit’s primary benefit is warmth. Buoyancy is a secondary benefit but one that may or may not be important to you depending on your swim background. Just because a race is wetsuit-legal (water temp at or below 78 degrees) doesn’t mean you are required to use one.
Based on the weather in St. Louis which I assume is similar to Bloomington, we’ve had a normal winter and no really warm days so far, so I would expect most participants would be wearing wetsuits.
Whether you need one depends on your cold water tolerance, swim ability, distance of the race (the longer the swim, the more the benefits).
If you decide to use one and you’ve never used one before, I would not recommend wearing it for the first time during the race. Try to practice in it before the race.