Hello Stijn,
The RadToolTip is intended to show the title with a higher priority than the explicitly specified content. This being said if you have set its content and at the same time you have set the ToolTip property, the control will show the value assigned to the ToolTip property.
This is logged in our database and we will soon provide the ability to determine the priorities, probably in the SP2 release.
All the best,
Svetlina
the Telerik team