A triathlon wetsuit is thick neoprene material primarily designed to keep you warm, although buoyancy and hydrodynamics are huge added benefits.
A tri suit or triathlon suit is an outfit made of thin polyester-like material (I’m actually wearing one right now, coincidentally, as I’m writing this just before going to do a workout). While a tri suit may provide some buoyancy and warmth, it’s very, very, minimal compared to a wet suit. The primary benefits of a tri suit are:
1. Hydrodynamics (if you’re swimming in a triathlon where you’re not wearing a wetsuit)
2. Aerodynamics on the bike and run (not a huge benefit on the run, but some potential benefit on the bike)
3. Comfort – When people say “tri-suit” they’re generally talking about a one-piece suit. This means you don’t have to deal with any chafing or rubbing around your waist, if that’s a problem for you with a normal tri jersey and pair of tri shorts.
4. They look pretty cool.
If you’re doing a non-wetsuit race, then yes, I’d say a tri suit is a must-have. If you’re going to have a wetsuit on (which you would definitely want to wear if they allow it–you would never opt for a tri suit in place of a wetsuit given the option), then you’ll wear your tri suit under your wetsuit, so it’s not going to do anything for you on the swim. Therefore, the primary benefit for a beginner triathlete would be comfort, but if you don’t suffer from discomfort when wearing normal two-piece triathlon clothing then a tri suit at this point is probably overkill.
One downside to a one-piece tri suit over two piece triathlon wear–it’s harder to go to the bathroom, and this is no small matter if you need to go in a hurry. For this reason you may see triathletes walking around the transition area with their tri suit unzipped and the top part hanging from their waist prior to the event starting.