Our girls are 22 months age difference and they fight like mad. However it got all the sleepless nights, potty training and bed wetting all done over the space of a few years. I thought they would play better but 2 totally different personalities and they clash constantly. I doubt it would be any different if one was a boy!
But at the same time I'm glad we had that short gap, I see folk with kids now and I doubt I could go back to the baby stage again now my 2 and do so much for themselves and sleep all night.
My twins and their older sister have a 13 year gap. Age gaps mean nothing.