I normally play 2000 points and I run a typical Necron army without a Ctan.
I have normally played with 2 squads of 3 Destoyers... I just recently tried the 5 Destoyers and 1 heavy Destroyer. I only use 1 cuz I only have one.
I have found that 5 Destoyers are a pretty hefty force against Strong Infantry and Light Vehicles... where as the Heavy Destoyer puts down the heavy tanks or Heavy characters.
What is your preference?