The answer to this question is a math problem. @maxim809 solved it correctly. For riders under 5w/kg, gaining 5w and holding kg constant will increase rider w/kg to a greater extent than losing 1kg body weight and holding watts constant. This holds true for any body weight and power level (obviously because we are talking in w/kg).
It is also true that while holding w/kg constant, the athlete with higher absolute power will always climb faster. For example, the 5w/kg rider at 75 kg will always out climb (at any gradient) the 5w/kg rider at 60 kg. See Climbing and W/Kg
So the interesting (and better) question is: at what absolute power level is it better to either lose 1 kg or add 5w (holding other metric constant) in order to climb faster?