I just ran some numbers, considering that energy damage is usually the Reflex-and-save-for-half kind vs. a 5 resistance.
At level 1, the bonus-to-save option is better if you get hit by spells that do more than 200 points of damage.
At level 5, the bonus-to-save option is better if you get hit by spells that do more than 100 points of damage.
At level 10, the bonus-to-save option is better if you get hit by spells that do more than 67 points of damage.
At level 15, the bonus-to-save option is better if you get hit by spells that do more than 50 points of damage.
At level 20, the bonus-to-save option is better if you get hit by spells that do more than 40 points of damage.
Maitre D