First of all you need to take into account the weather conditions and aerodynamics. The body is 80% of your drag, so wearing a tri-suit is always going to be the best option. By combining good position and clothing you could save up to 30-40 watts, which could equate to 90secs-2mins over a 40km course.
When it comes to suits always opt for a sleeved version – the shoulders are a very sensitive area so by wearing an aero suit you’ll cut drag. You’re going to save around 15-20watts with an aero suit compared to shorts and a jersey. The brand Endura, for example, has a tri-suit with different panels that are designed to reduce drag but also allow you to move more freely. My own personal preference was to use a full-sleeved skinsuit one size bigger so it allowed me to move easily.
When it comes to the weather, always make sure you layer up well. But make sure you keep the base layers under your suit rather than over them.