We've been to both the HDDR and the SoA (luau) ... while we liked both shows, we would return to SoA before HDDR.
We experienced the HDDR in August 2008. We're a pretty laid back group - easily amused, not hard to please, and quick to laugh at anything silly. but the only one whose attention was really held by the "show" was the 16 MONTH old. (He was enthralled - so it was worth it just to see him staying over with balcony with his chin propped in his hand and his eyes glued to the stage.) The food was good - not gourmet but good. That said, it was typical of the BBQ-type-fare that you can get at other places around WDW (WCC for example).
We experienced SoA in August 2006. Looking back, this meal was lighter and more refreshing compared to the heavier fare of the HDDR. It also was different from the other TS meals we had. In terms of hte "show" ... as others have said, there were part of the storyline that dragged a bit - but these seemed to occur duing the meal portion when people were busy serving themselves, eating, and chatting with those at their table. (Not a bad thing actually, as you got to enjoy your meal without being totally distracted by the show.) The tradtional dances and the fire-dancer were awesome - and seemed to take place during lulls in the meal service. (Note: We did go to the latest show, so that probably helped with ambiance and also eliminated some of the heat.) We had my younger cousins with us that time (girl aged almost 16, boy turned 13 during the trip) and they were totally blown away by those parts of the show. (We have pictures of all of us looking wide-eyed and awestruck during different parts of the show.)
For us, SoA was the more novel experience - none of us have been to Hawaii and leave in the polar opposite of a tropical climate. I don't think you can go wrong with either - pick the one that you think your family would enjoy the most!