First question to ask is "Am I allowed to wear a wetsuit in the race?" Different race organizers follow different rules, and there are often wetsuit regulation differences for pros vs. amateurs. USA Triathlon rules:
Each age group participant shall be permitted to wear a wet suit without penalty in any event sanctioned by USA Triathlon up to and including a water temperature of 78F. When the water temperature is greater than 78F, but less than 84F, age group participants may wear a wet suit at their own discretion... ...wear a wet suit within this temperature range shall not be eligible for prizes or awards. Age group participants shall not wear wet suits in water temperatures equal to or greater than 84F. The wetsuit policy for elite athletes shall be determined by the USAT Athletes Advisory Council.
If you are going to wear a wetsuit in a triathlon, sleeve or sleeveless is often more a question of personal comfort and feel, so it is a tough call on exact water temperature (there are other considerations to make when buying a triathlon wetsuit). As long as the wetsuit fits properly, 99% of the time a swimmer will be faster in a full wetsuit in any wetsuit legal race; that 1% is often due to trouble getting the suit off, and that just takes practice. 70F water - I'd be OK in a full wetsuit, but if it gets a few degrees warmer I might opt for a sleeveless. I tend to get cold easily, so I like a full sleeve - my personal preference. I know swimmers that hate the sleeves, they have the feeling that the suit is trapping them, it limits their mobility, and they cannot feel the water as well. All of this is valid, but it may be due to a poorly fitted or a poorly put-on wetsuit, too.
There are cosmetic and minor brand-specific feature differences between wetsuits at similar price points, but in general if you pay $500 for any brand of wetsuit you are getting a suit more alike than different from another brand's $500 wetsuit. Wetsuits use very smooth, very "stretchy" neoprene. As the suit price goes down, the neoprene used ends to be a less expensive and less "stretchy". This leads to a little more durability, but a little less performance.
I notice the difference if I wear a less expensive wetsuit; it is harder to swim, and it takes more of my energy to perform at the same level. Almost any triathlon-specific suit should meet your needs, but the ones with the highest suggested retail price are probably better performers than the ones with the lower retail price.