As I'd already touched on Sigma's quality of brushes, I don't think it's much different if not better with these two aside from the duo fibre brush did have a tendency to shed a little during cleansing. An added bonus is that they come with brush protectors too which is a hygiene bonus - not only from other brushes but from dust collection during storage. Any who I'll go on to talk about the brushes individually:
F50 - Most people use duo fibre brushes to create 'airbrushed' effects with their foundations by stippling the product onto their faces with it, however not only do I prefer to use my hands to apply my foundation (judge me all you like) but I feel that the F50 isn't sturdy enough for controlled stippling unless you're planning on ever so lightly grazing the skin with the brush. On the other hand I've found this duo fibre brush a saint for applying highlighter and blush.
F35 'Highlighter' brush, the tear drop shaped brush does do a highlighting job well getting onto tight areas like the bridge of the nose or utilising the brush and sweeping not only under the eyes but brushing across the cheekbones too, I found this brush also works well with concealing.
To order some for yourself go to Sigma's Website :)disclaimer: Although these were sent to be from Sigma, I wouldn't review these unless I truly thought they were worth the effort