If you're riding in consistently hot weather, underarm and side mesh panels are an excellent choice. This design extends mesh ventilation from your underarms down along your torso sides, creating a larger cooling zone than underarm mesh alone. It's ideal for hot summer rides, especially if you tend to run warm or are pushing hard efforts where heat builds quickly.
One thing to keep in mind: the side mesh is more visible than underarm-only panels, so if you wear a base layer underneath, it may show through. This isn't a problem for most riders, but it's worth knowing if you prefer a cleaner look.
This level of ventilation is best for genuinely hot conditions. If your rides start cool and warm up later, or if you ride in variable spring weather, the side panels might let in too much chill air early on. For those situations, a jersey with just underarm mesh gives you some cooling without overdoing it.