I'd go with the 20/20 60 energy offhand set. Once things start dying you will never be at a loss for energy thanks to Soul reaping. You'll never get to use that extra 10.
*edit*. you can always use both you know. use the 20/20 set, and if you run out switch to the other.
personally I like 20/20 over 15/-1 because I will never run out of energy when things do not die.
It really depends on the situation. Clearly if you have a high damage group with few defense/support/healing characters, you want 75 energy because things will die faster.