Leaving aside the secondaries, my thinking was as follows:
Controllers depend on their control set for damage. AoE damage is generally the most important. They also can depend on their secondary for additional defense.
Dominators depend on their control set for defense, and on their secondary for damage.
This makes me consider that I want to run the higher-damage sets as Controllers, and the higher-defense sets as Dominators.
I consider the higher damage sets to be:
Fire (natch), Illusion, Plant, Electric, Darkness
The lower damage sets are:
Gravity (AoE, it's good single-target), Ice, Earth, Mind
Ice is a bit of an odd case... at low-damage you want to run it as a Dominator, but its main defensive strength is its -Recharge debuff, which really synergizes well with a debuff secondary so that you can lower enemy accuracy, damage, and attack rate, while healing what damage does come through. So a character like an Ice/Rad is a very strong defensive combo even if you never use anything more than -Recharge powers and the AoE Immob for damage. However, you're low on damage. So I think that the best option for Ice is to run it as a Dominator, with the Dark Assault secondary... you stack -ToHit with -Recharge to lower incoming damage, get mez protection so the slow but constant incoming damage doesn't mez you, and a self-heal with which you can counter the incoming damage.