Err on the side of comfort
A good swim is a comfortable swim. If you are too cold, too warm, chafingany of those things will impact your stroke technique, breathing, and confidence. A couple of common easy fixes for comfort:
Chafing - Wetsuits are great. They keep you warm and help you go a bit faster. They also create nasty chafing and scars if you don't use some form of personal lubricant such as Body Glide or Sport Slick.
Temperature (cold) - If it is cold, don't hesitate to slip a second cap underneath the one the race directors provided for you. That second thin layer of latex will keep your dome nice and warm.
Temperature (warm) - Everyone assumes that you should ALWAYS use a wetsuit. If the water temp is quite warm but not too warm to make wetsuits illegal, consider using a sleeveless (if possible) or, GASP, take it off altogether and just use a swimsuit like they did in the prehistoric tri-era. I think that era was called the tri-a-zoic.
This Information Brought to You by Konrad Ribeiro