If you are just starting off, go for the cheap nylon brushes.
If you have some pratice and know how to use and clean the brushes for the differnt types of paint then go with sable. It costs a lot more but there is a big difference in the finish.
Bigger the surface, bigger the brush.
Have cheap nylon burshes on hand for things that will destroy any brush.
For painting 95% of the surfaces you'll want the flat brushes with the square ends. I like them to be a bit long as well as thick. Other 5% i use the rounded brushes that end in a point, these are good for detailing.
If you have little experence with model builing using paint brushes, use the cheap nylon brushes and buy one or 2 of each type. Build a few models and you will discover what "you" need.
Mongol General: Conan, What is best in life?
Conan: To crush your enemies, see them driven befor you, and hear the lamentations of the woman!