The first question to ask yourself when choosing your fleece is the following: what will you use it for? What type of physical activity and what level of intensity?
When making an intense effort, like hiking up a steep slope, you sweat more. To avoid being soaked inside a damp fabric, it's important to choose a breathable fleece, i.e. a fleece that can eliminate the water vapour generated by your perspiration.
To make it more breathable, there may be zipped openings on the fleece so that you can regulate the ventilation according to your physical effort: half-zip or full zip at the front...
For more moderate efforts, therefore, you will not warm up as quickly and you will sweat less. Consequently, the level of breathability is not as important. In this case, it would be preferable to choose a thicker fleece with more insulation.