All things being equal, I think 50/50 custody (or as near to that as possible) is probably the best.
In our situation (@laineybug is my daughter), it was best for our girls that their father have primary physical custody (we have joint legal custody). That way, they could stay in the only home they’d ever known (a very nice house), with a good parent who was also able to afford that home. If they’d lived with me, it would have been in a tiny apartment in a not-so-good neighborhood. We made sure I was still a huge part of their lives…for many years after their father and I divorced, I picked them up from school every day, and stayed with them in their house until their dad got home from work in the evenings. (Luckily, we are very effective co-parents.) Now that they’re older, I still go over there once a week to hang out with them, and of course they come stay at my house often, too.
Long story short, I don’t think it matters so much who the kids live with, but it does matter that the non-custodial parent be a regular presence in their lives.