If you need to specify the target type of your style explicitly, you should favor the syntax that uses the x:Type markup extension. The reason for this is primarily that it guarantees consistency in your XAML code, but also that the use of the Typename-as-String alternative may lead to problems.

  ✘  <Style x:Key="MyCustomButtonStyle" TargetType="Button"/>

  ✔  <Style x:Key="MyCustomButtonStyle" TargetType="{x:Type Button}"/>

This recommendation also holds for other attributes that expect a Type value, such as ControlTemplate.TargetType or RelativeSource.AncestorType.