Yeah its a real hairsplitting challenge you've got there. The age groups you've got are whats going to make the decision hardest.
Usually the younger kids are said to be far better catered for at Dreamworld with both ABC Kids and Dreamworks, whereas the teenagers are probably going to enjoy Movie World's big rides.
The correct answer for me, would probably be 'do both' as you'll find each group will excel in different areas. As you've seen - even amongst folks here, there are split opinions on which one to do.
If you can only manage one, i'd involve the teenagers in the decision. The younger kids might not have as much fun, but tend to be less fussy - whereas the moody teenagers are far more difficult - its better to aim to keep them pleased!
Whether you do or don't get to Dreamworld - remember they have Moonlight Markets on on Friday and Saturday nights. No rides, but the park hosts a food market, outdoor cinema, and their main street store, candy shop and lego store are open for the duration - and even if you don't 'go' to dreamworld, the markets are worth a drop in for dinner with plenty of choices. (entry is $2 for adults, but passholders, or those already in the park can attend the markets free of charge).