@Fer
The quill is made to have a large belly so that it can hold more water. It's for painting large areas.
The round brush is for details. Actually the quill can be used for details as well since the point is sharp. However, if you use the quill to paint details, it will be mean you're going to pick up lots of paint and you may not want or waste that much paint. Hence for painting details, the round brush is more suitable.
@Fer
The quill is made to have a large belly so that it can hold more water. It's for painting large areas.
The round brush is for details. Actually the quill can be used for details as well since the point is sharp. However, if you use the quill to paint details, it will be mean you're going to pick up lots of paint and you may not want or waste that much paint. Hence for painting details, the round brush is more suitable.