I recently went through the somewhat painful task of finding a triathlon wetsuit. After doing a fair amount of research online, getting opinions from more experienced friends, and trying on a few suits, here's what I've determined:
1.) Do I need a wetsuit?
For the typical age-group triathlete the two biggest benefits of a wetsuit are insulation and buoyancy.
USAT rules allow the use of a wetsuit for water temperatures at or below 78 degrees. You can still race in a suit at temps between 78.1 and 83.9, but you will be ineligible for any awards. For temps 84 degrees and above suits are prohibited.
When purchasing a wetsuit consider the temperature of the water in your intended races. Ironman series and other well-organized races will post the expected race day temperature on their websites.
If you're racing in cold water, then the additional insulation can be a blessing. However, a full suit on a warmer swim can be a miserable experience. You can always pop the neck open to let in some water to help cool down, but I suspect that you could forget to do this on race day. So you need to consider where the majority of your races are going to take place. For races under 70 degrees a suit is highly recommended.
The concept of insulation doesn't just apply to water temperature. There's a lot of contact in a mass start swim. Chances are you be crawling over someone or being crawled over. Your wetsuit will protect your skin from all that incidental bumping and clawing. It's also nice to have a little shielding from the not-so-pristine waterways you might find yourself swimming in. I hear that for races like Louisville you'll be splashing through boat fuel and other funky pollutants. A wetsuit provides a nice barrier - at least psychologically.
I think one of the biggest benefits of a suit is buoyancy. I attended a swim clinic where they filmed us swimming. The underwater shots were quite telling. Nearly everyone's legs sunk deeper into the water than their upper body. Ideally your body should be parallel with the surface of the water. This would provide for the least drag. However, keeping your butt and legs at or near the surface can cost you a lot of energy. The wetsuit helps lift you into that correct position - the biggest boon to your swim.
Additionally, most triathletes I know consider the swim to be the most difficult and intimidating part of the race. If you get tired during the run or the bike you can always just stop. During the swim however, you don't have the same option. The wetsuit helps keep you afloat.
2.) Full vs Sleeveless
If you wear the suit correctly (high and tight) then a full suit will be faster. This is primarily because there is less drag across the shoulders and arms (the parts that are trying to break through the water). The problem is that shoulder fatigue can more easily set in because you're having to stretch the rubber surrounding your shoulder with each stroke.
If you trained in your suit everyday, then this probably wouldn't be an issue. But most amateurs I know train in a pool without a suit, so this extra shoulder effort on race day can take a toll. If you decide for a full suit it makes sense to get a really "stretchy" suit to minimize the impact. However, this more than anything else will drive up the price. The "stretch-factor" is the main difference between an entry-level suit and a high performance suit - at least as far as the typical age-group athlete is concerned.
Sleeveless suits don't impede your shoulder movement (as much). So the stretch factor is not as important. Eliminating this factor can be mean a big dollar savings. Not only are sleeveless suits less expensive because they require less material, but considering that stretch is less important you can also get by with an entry-level suit.
DeSoto has a line of full sleeve suits that come in two-pieces, supposedly to help reduce shoulder fatigue. I've seen a few good reviews about this line, but I don't know of any objective reports on the benefits.
Here's my theory: If you are going for a full suit seriously consider the higher-performance suits and be ready to fork over some cash ($500+). If you choose sleeveless then just go for an entry-level suit ($200-300) - you'll still get the major buoyancy benefits.
3.) Correct fit is critical.
No matter what level of suit you get, it should stick to your body like a second skin. There should be no pockets of air or loose spots. You must put the suit on high and tight.
What did I buy?
I scoured the Internet looking for a great deal, but didn't find anything but closeouts that wouldn't fit. Obviously spring is the wrong season to be looking for a deal on triathlon gear. I lucked out and found a closeout sleeveless Orca Predator 2 at Big Shark, a local athletic shop. It was a new suit that never made it out to the rack. Discount Price: $150. I'm still looking to get a full-sleeve suit, but this was a nice addition to my equipment arsenal at a good price.