If we are talking Capped Resistance and 0.00% Defense vs Capped Defense and 0.00% Resistance?
I'd go Resistance. As it's a steady, predictable damage reduction every time you're hit.
While Defense is VERY attractive, it's binary. You're hit or you aren't. If you have NOTHING underneath your Defenses, it stops becoming quite so attractive.
Even with Tank HP, it becomes a Brute/Scrapper type experience.
"I'm in my happy place!"
"I'm in my happy place!"
"I'm in my happy"*SMOOTCHING FLOOR!*
You're basically living at the whim of the RNG Gods.
Resistance, you die in a steady "This ain't gonna end well." scenario.
Defense, you're sitting there, blown into salsa, wondering "What happened?"
If we're talking mixed mitigation types...It becomes VERY situational.