Right -- I have read up on the powersets. Please indulge me in some napkin math here.
Dark (which I think is superior to Rad because of it's larger Radius) gives roughly 22.5% Def. Maneuvers is about 6%. Shadowfall is another 8.5%. Call to Arms Enhancement is 5% roughly 42%. This is almost capped.
Cold -- Powerboosted Shields 25%, Arctic Fog 6%, Maneuvers 6%, Call to Arms 5% = 42% (and I presume people take Medicine with this)
Radiation -- Rad Infection 30% (small radius), Maneuvers 6%, Call to Arms 5% = 41%
So in short---about 10% of the time they take 100% S/L damage.
Thermal - Shields 24%, Expedient Reinforcement 10% = 34%
Nature - Sport Cloud 25%, Manuevers 6%, Call to Arms 5% = 36% Def and Wild Growth 18% (some down time) + Exp Reinf 10% = 28%; so get hit 15% of the time for 70% of the damage.
The Def based powers seem far better to me. The resist values seem too low to compared when comparing to how much less dmg gets through when Def capped (and why I thought tankerminding would be used with those). I suspect I am missing some key powers or synergies or set bonuses here. I used the non set IOs.