The decision of which brush to buy for paint application could lend itself to be a bit of a challenge. The array of brush filaments and firmness should be paired up with the paint or product you are using(we can delve into brush construction at a later point).
As an example, if you are laying down a coat of a waterborne polyurethane, you would probably want a very soft synthetic brush in order to minimize the appearance of brush strokes. For the higher solid wall paints you may want to use a stiffer filament brush for better control and application.
Oil Based Products: For oil based products you can use synthetic or natural bristle brush. The order of natural bristle firmness goes from the stiffer Black China, to White China, and then to Ox Hair. Do not use a natural bristle brush with any waterborne product because the natural bristles will absorb moisture and you'll end up with a broom for a brush.
Latex/Waterborne Products: For latex products use brushes made of synthetic filaments which could be Nylon, Nylon/Polyester blends, Chinex, and more. Each blend or filament has its own firmness characteristics. These are designed for use in different environmental conditions, different products, and for different applications. The unspoken guide to picking the right brush is based on your comfort and feel during the application process. We tend to pick the stiffer filament brushes because they handle the heavier bodied paints better while offering a sharper cut line.
If you are planning on painting a great deal...don't skimp when buying a good brush! Even good brush companies make poorer/entry level quality brushes that won't last or will make your efforts look less than desirable.