I'll admit I know nothing about D3, but at least in EvE, there's a concept of "Effective HitPoints" (EHP), which is HP divided by the hole in your resists: {Real Hitpoints}/(1-{resistance})=EHP . I would think it applies here, too.

Of course, similarly, if healing isn't resisted, then EHP gained by resistances is better than EHP gained by base HP.