They help you swim easier (same speed but less energy cost) or they help you swim faster (faster speed but same energy cost). Oh yeah - they also can keep you warmer (but you do not want them to make you hot; that is dependent upon the water temperature, so you have to figure out what works best for you - and, of course, respect the wetsuit and water temperature rules of your particular race).
What is the primary way that a triathlon wetsuit helps you swim faster? Floatation - drag reduction - the wetsuit should put your body in a great swimming position (horizontal) regardless of your head position or regardless of where your eyes are looking. Your head position generally controls your body position, and a good head position generally results in a good body position - but with a swimming-specific wetsuit on your body, your head is taken out of the equation. The wetsuit material should also offer a slicker surface than your skin, further reducing drag.
Dollar for dollar, wetsuit reviews often state that the top end of any one brand of wetsuit is more alike than different from other brand's top end suit, and I agree. Each price level of triathlon wetsuit is relatively equal to its competitor. They tend to use the same type (or a similar type) of neoprene across any particular price level, and the higher price levels use a neoprene that is more flexible or somewhat more slick.
Once you decide about how much you want to spend, get to a shop (or visit several shops) that sells various brands and that will allow you to try on their wetsuits. Try on wetsuits in your chosen price range.
You may want to try on suits at price points above and below, too, but you really do get what you pay for with a triathlon wetsuit, although at the high-end part of what you are paying for is the newest, best, greatest gizmo in the line. Is it better than a lower priced model in the same line? Yes, in some way it is, but once you get near $300 for a suit, how much more "speed" is another $100 or $200 going to buy you? Not a lot, but if the difference in your race finish place is vital, then buying the top-end is the way to go. If you are a mid-pack triathlete, a top-price triathlon wetsuit will not move you up any higher in the standings than if you bought a mid-priced wetsuit. At the very least, I believe that the lower end, multi-use price point wetsuits are not the best choice for a triathlete unless they are not worried about a better swim time. You have to spend some minimum amount to get something that will work, and in a full-sleeve design that is around $250-$300.
Different brands have different fit tweaks and fit design thoughts, and they can feel different. You may be looking at two great choices. One may be designed to feel tighter or snugger, while another may be less snug, but both could help you swim better. The one that feels the best to you is likely to be the best one for you, while another triathlete might feel better in the other one - and that one is the best for them. I bet that either of you would be equally fast in either one, but as long as you are spending the money, you might as well get the one that feels the best to you, right?
It is hard to make a bad choice if you are willing to take the time to try on the wetsuits, but if you cannot try them on, be sure to be very accurate with your body measurements and the recommended sizes from the manufacturer. If you cannot get to a triathlon shop, make some phone calls or send some emails and take the time to ask about the sizing parameters of any brand that interests you.
What features should I look for when considering a triathlon wetsuit purchase? We'll look at that on page 2.