Mix em. Rads debuffs are so ridiculously high you won't need more than one of them. Irradiate alone does 40% or something silly like that. Basically when the GDN kicked in and all Defense values were halved the Defense Debuff values weren't adjusted to match. So you shouldn't need more than one Rad really (stuff him full of Achilles Heel -Resist PROCs as well).
A Emp/Dark and an Emp/Ice combo would be interesting for Tentacles & Night Fall + Ice Storm or Blizzard (defenders get the Blaster versions of the Ice Rains). Slow Death by Orange Numbers!!
You'll likely have better luck in the Defender forum.
Currently running a trio of empathy defenders, made them after seeing a post somewhere else on how good Emp/Sonic can be.
Sonic debuffs a mobs resistance, so subsequent hits do more damage. While playing with my current trio I wondered if having a rad trio (secondaries debuff defence making subsequent attacks hit more often) would be better... or would mixing the two be better still.
So my questions are for a proto 'Super Team'. Will I be better off with a mixture of secondaries or sticking with just sonic or just rad? Are there any other interesting secondaries to look at?