What decides the choice
The most important line on a pair of sunglasses is the UV rating, and it is separate from how dark the lens looks. UV400 means the lens blocks light up to a wavelength of 400 nanometres, which covers both UVA and UVB. A dark lens without that protection can be worse than none at all, because your pupils open wider behind it and let more unfiltered light in.
Darkness is described by the filter category, from 0 to 4, under EN ISO 12312-1. Categories 0 and 1 are for overcast days and fashion use, category 2 suits moderate sun, category 3 is the general choice for bright sun and holidays, and category 4 is for high mountains and glaciers. Category 4 lenses are not suitable for driving, and the frame or the label has to say so.
Polarisation is a separate feature again. It cuts glare reflected off water, wet roads and snow, which makes it valuable for driving and fishing, but it can make phone screens and some car displays harder to read. Beyond the lens, judge the frame on fit: it should sit without pressing behind the ears or resting on the cheeks, and spring hinges and adjustable nose pads help a pair stay comfortable through a whole day.