There's no difference in quality between the Fluenta, Elite Mirror and Elite Smoke Collections, and Digit Collection. It all comes down to personal preference.
Much like shoes and purses, each Artis Brush collection is designed to express a different sense of fashion, lifestyle and aesthetic. For example, the Fluenta brushes design is inspired by the shape/movement of water flowing over an edge, which creates a deeper curve than the Elite Collection. The Elite Collection shape is a design/artistic example of Deconstructionism, whereas the Fluenta Collection is artistically a more Modern, fluid design statement.
All Artis Brushes are constructed with ease of use and ergonomics in mind. Some Design Collections are deliberately designed to look more curvy or delicate, while others offer a more uniform/flowing size, shape and aesthetic. If you feel more comfortable with a Design that is more firm and uniform with a deeper curve at the end, you may prefer the Fluenta Collection. If you like something more delicate looking, the Elite Collection may be more pleasing.
There's no difference in fibre bundle performance between the Elite Mirror and Elite Smoke collections. While the Elite Mirror Collection was designed to simulate a liquid chrome look, the Elite Smoke Collection has a darker smoked chrome finish and grey grip pad.