HDDR is great. Fantasmic is fantastic. I don't consider them mutually exclusive.
Fantasmic is free with park admission--thus, we go almost every time. It has changed slightly over the years. Since this doesn't cost you in $ or credits, I'd definitely do Fantasmic. Whether you do the dinner package or not is really a different decision.
HDDR is well worth doing. However, consider whether you want to spend the $ or credits. Since it has been several years, I'd go for it.