What suit is appropriate depends a bit on:
- Obviously, what the water temps are -- including any thermoclines
- Length and number of dives? (More dives and longer/deeper dives tend to make you colder)
- What is your natural tolerance to cold?
- How active are you during the dive? (slow and deliberate is colder than the swim meet approach)
- How much can you warm up (or get further cooled down) between dives? (Can you bask 90 degree heat or will you be on a moving boat?)
Personally, I try and dive with just a t-shirt and shorts if the water temp is 80 or higher. I often wear a beannie as well. On that same dive, my brother will freeze in anything less than a 3/2 full length. I have seen people in those conditions use a 7mm. So it really just depends...
Personally, if I already owned a shorty, I might start with that. But if was buying a wetsuit for the first time, I would buy something like a 3/2.