What Base Layers should I buy? Cooling or thermal? And during what seasons should I wear them?
The primary role of any base layer is to control the microclimate next to your skin by moving moisture away from your body. The secondary role relates to either trapping or removing the metabolic heat your body generates.
During Summer you want this heat to be able to escape through a combination of removal of trapped moisture, and airflow for evaporative cooling. Ice Dry fabric with zoned mesh panels uses capillary action to draw moisture off the surface of your skin transferring it to the surface of the fabric. This is done without the addition of wicking chemicals, that only have temporary effectiveness. Guaranteeing you will as be as cool and comfortable at the end of summer as you were at the beginning.
Winter base layers should still transfer moisture off your skin, but the secondary role is to trap the heat, holding it next to the skin acting as your first thermal layer. Merino wool for example is excellent at this, with the ability to draw moisture without huge loss of thermal ability.