{x:Null} vs. Transparent?

Transparent will create a brush that is initialized to a transparent color, null will set the property to null, this means that the destination property has not an brush attached. In WPF it's often important to set a brush to an element. If you for example want to track mouse downs in an element, you must set a background. If you don't want to set a solid color (make it opaque), you can use a transparent brush. This can be done with the string value "Transparent".
The difference lies in the manner, how the property will be set. If you assign null for a brush-property, the property will be set really to null. If you set the string "Transparent", the default value-converter that converts string to brushes converts this to the Brushes.Transparent brush.

Short version: {x:Null} sets the destination property to null. "Transparent" sets the destination property to a transparent brush.

Both are setting the local value of the Background property. The former sets it to null and the latter sets it to Brushes.Transparent.

There are a couple of important points to be aware of:

• Setting the value to null is not the same as not setting it at all. Since dependency properties obtain their effective value from multiple sources, setting a local value (even if it's null) can take precedence over values potentially sourced from elsewhere, such as a style or animation.
• Another option for controlling hit test visibility is the IsHitTestVisible property. This property allows you to control hit test visibility regardless of the brush with which the UIElement is rendered.