I am about to compete in my first triathlon in September in the frigid waters of Monterey Bay California and are looking into the Tri gear I need. I am unsure whether I get a tri-suit or a wetsuit or if you get both and wear one under the other.
If funds are limited, buy the tri suit and wear it under a rental wetsuit.
IMHO, the tri suit is a bit of a luxury, but I would not want to swim in Monerrey Bay without a wetsuit.
Depending on the water tekp a good wetsuit will keep you warm, help you conserve energy on the swim and make you faster. You woud wear either a trisuit (one-piece) and tri shorts and racing top under the wetsuit so when you peel the wetsuit off at T1 you're ready to go.