I am going to agree and disagree with the above posters. I agree that it is usually not the best choice to use 2 TS credits for one of the dinner shows. This is mainly because you are getting Category 2 or 3 seating which is usually in the back or on the side.
However I disagree with those who are saying the Luau is terrible. For every person who dislikes the show or food, there is someone who likes it. I don't think any of the dinner shows could be called terrible. We loved the food at the Luau and the show was fine, certainly not a Broadway-caliber performance but it was entertaining. Most people rave about the HDDR but we didn't care for it. Again, I wouldn't call it terrible but it wasn't for us.
If it would be inconvenient to use the 2 remaining TS credits for two more meals, then I would use them for a show instead of wasting them. If you have younger kids, then maybe the BBQ or HDDR would be a good choice. I would say the Luau is probably best for kids over the age of 6. My daughter was 8 during out last trip and she enjoyed the show and it kept her interest.