The lower the number...the darker the tint. The percentage number represents how much light the film will allow thru. So a 5% tint film...only allow 5% of light to filter thru...a 35% film...will allow 35% of light to filter thru. This means that the 5% film will appear darker than the 35% film.
How dark the film you use is personal (and legal)...no one can tell YOU what film looks better. You need to look at cars with tinted windows and choose the one you like...or take it to a tinting vendor and let them possibly hold up some of the film to your car to see which one you prefer.
Good luck.