Just thinking from my pov.
Pfb had me for 3 years before PSB arrived, in those 3 years he had total 1-1 attention from me, we went swimming a lot also the park and holidays. I just had more time for ds as an only children
When dd was born I had to spread my time with the 2 children.
I wouldn't say I have a stronger bond for pfb as I love my children for different reasons and I have a different bond.
But do you think first born children have a better start in life.