The wetsuit would help you on the swim, it adds a bit of buoyancy and keeps you warmer. The first time I did the Chicago tri, I was freezing the first year when I did the swim without a wetsuit and the water temps were 67 or so. Having a swimsuit for that swim would have shaved about 10 minutes off my swim and made it much more comfortable during the race. The problem is that it's hard to predict water temperatures since rain before the swim will cause the lake temperatures to go down a bit.
You can always rent a wetsuit for 40 or so during that weekend or get an used wetsuit for around 100-150.