The Neato Combo Brush is a combination of rubber blades and bristles whereas the Blade Brush only use rubber blades.

The combination brush has 2 advantages over the blade brush.
The first one is that it significantly cuts down the blade noise while the brush is rotating and hitting the floors. You will feel the difference if you try the combo brush, the robot gets a lot quieter.
The second one is the ability of picking up pet hair. The combo brush improves the performance on picking up pet hair. So, if you have dogs and cats at home, the combo brush is a great upgrade from the blade brush, definitely a must have.