Competing in my first triathlon, do I need to invest in a wetsuit?

The water temp will be around 70 degrees. Is it necessary to get a wetsuit? I have already doled out 110 for a tri suit.

What type of wetsuit do I need to buy to compete in the Chicago Triathlon on August 29, 2010?

I see a lot of people wearing shorties or 3/4 types in last year’s photos, but I wonder if I could get away with a full suit – then I could wear it in a wider range of water temps. Any thoughts? Does anyone know what the water temp is in Lake Michigan that time of year?