Instead of being one buff or the other, there's this in between alternative that I think is much more flexible:
Eclipse could have stacks for each buff, with a combined total maximum of 5. Each stack would be 20% of the maximum buff given by the skill.
On cast, the damage buff will gain maximum stacks. If held while active, one stack from the light buff is lost (keeping it at 4 stacks or 80% of maximum buff) and one stack of dark buff is gained (getting 20% of maximum damage reduction). It would work viceversa if the initial cast was held.
It's almost like toggle but it would allow Eclipse users to have more than just one or the other, adding the possibility of hybrid buffs from the skill.