Not exactly sure how much more def you loose with zerk 2, but you definitely get more patk, more atk speed, more run speed... But I suppose you also loose more. But zerk is for offensive chars, and who of these cares about def anyway... ;)
Zerk 1 = lesser increase, lesser decrease. Zerk 2 = more increase, more decrease. It's kinda same with all spells, if they're higher, they have more power/effect.