Ok, DataTree was only added to make life easier for scripters. It basically removes the constraint on IGH_Goo, allowing you to use regular data types. However if you’re in VS then you should always use GH_Structure.
Just be sure to assign your tree to the appropriate output using DA.SetDataTree();, rather than DA.SetData(); You must also set the access level of that output parameter to tree in the RegisterOutputParams method.