Both sights are very high quality sights. It may depend on the use you plan for it. The Copper John sights have long been known for their strength and optics. The G5 is known for its precision and good looks. Both sights posses high quality components. Both should be strong and reliable. They both are easily adjustable and good optics. The question is when you look at them which appeals to most to you. I don't mean to be flip but beween these two sights I don't think there is a wrong answer.
It is like comparing a tricked out 4X4 truck to a hopped up Porsche. They are both great pieces of machinery. But one will usually appeal to an individual more than the other.