As others have said, there are advantages and disadvantages to everything.
I have two DDs who, at the moment, are very close... they even claim to love each other more than they love me and truly are best friends. the thought of either of them not having the opportunity of having that relationship is quite sad... so the benefits of having a sibling can be amazing.
I have a brother, who I am not massively close to, but we get on and I am very glad I have him. Despite our different lifestyles, I wouldn't be without him and he does certainly enrich my life greatly.
But DH has two siblings with whom his relationship is more difficult. He barely speaks to them, and so I am not sure siblings necessarily enrich his life.
Personally, I had DD2 for the benefit of DD1 - I wanted my child to have a sibling, but that decision was right for me. And although she benefits massively from being an icon in her younger sister's eyes, there are also ways in which she misses out... I think we are harder on her compared to her younger sister and so on...
So, there are strengths and weaknesses to both sides and everyone has their own view and baggage that they bring to this debate, you have to do was is right for you and your family, and unfortunately as no-one else knows your situation, only you can decide that. Sorry that is not much help, but good luck with it!