Long tails aim to control the airflow over distance and guide it smoothly onto your torso, but they rely on meeting your back; short tail (aka stubby) designs give you great freedom of movement because they’re designed to be efficient while independent of your torso. Because of the less extreme positions used in tri compared to time trialling, stubbies are often more suitable.
>>>Aero helmets for triathletes – how and why to use them>>>
For a long time designers pursued slim shapes with minimal frontal area so the helmet hits less air. But recently, some brands have moved to wider shapes in order for the helmet to integrate with your shoulders, though this relies on a lower position so that the helmet and torso work together.
Surface features such as dimples (Louis Garneau) and trips (Lazer) are designed to encourage ‘laminar’ airflow, where the air follows the shape of the helmet. Others (POC) go for the smoothest possible surface.
Remember that the helmet forms part of a bigger system and the rest of it is you, so your aero riding position and physiology (shoulder width, back length) are essential considerations. The very best method is to book some wind tunnel time, where you can hone your position and try a number of helmets to evaluate. If you order a helmet, try it on at home while sat on your bike (ideally on a turbo, pedalling lightly) and have someone take photos of you from the front and side. It’ll be easy to see if the shape integrates neatly or not, and especially if a long tail is stuck up in the air causing drag.