Really depends on the chip, as there are many different chips made from each company
but as long as they have (or there are) bugless drivers for their chips, I'm content
as for current onboard ethernet, they should all be the same unless you are utilizing your computer with some super fast connection or are using it as a dedicated server - then I wouldn't use onboard
if I did have to choose one, my preference would be Intel as I had great experience with their NICs
I always think separate NICs are better (as in performance) than their onboard counterparts (I could be wrong though)
my old mobo doesn't have onboard lan and I would disable it if it did