Ok that might be stupid thinking but...
...in most cases added effect from set bonus will be free from MP used. To get advantage (in mp efficiency) from getting that 2% more damage from nonaf3 you need a mob that will need exactly one less nuke than with af3+2 (ie: same amount of nukes with af3+2 will left it at few% HP left) Whenever the mob is x-nukeable with the same amount of nukes with both sets then set bonus proc will in most cases shave you one nuke. In that case having more set bonus chance is more chance of having free damage. Obviously the difference is very small but so is 2% damage. Best would be to have both sets ofc and use oneiros+valkyrie whenever those few mab more will shave you one nuke to kill a mob otherwise it has not much sense to use it.
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