I am sure tongue and groove would be stronger if a good tight fit. It might also compensate for any slight imperfections on the adjoining edges, if there are any.
I think the PU glue used on the rudder was Wurth. However I have subsequently used this one with similar results:
http://www.wudcare.co.uk/fastgrab.htm
The PU glues seem to work best on tight fitting surfaces. Epoxy may have better gap filling qualities.
Hope that helps.
David