Steff,

I use both methods of construction depending on the configuration of the wing. If the wing is sheeted between the LE and Main Spar I glue the shear webbing to the back of the spar, if the wing is open between the Main Spar and the LE as on my "Bingo" I use the I-Beam.

I have been building this way for more than fifty years and have never had a wing failure. Using the D box construction (sheeted between Spar and LE) is much easier and faster to build.