I don't know enough about the D300 to compare but I would suggest the argument only applies to people buying their first DSLR. Those looking to an upgrade, either Nikon or Canon, are hardly likely to ditch all their lenses and accessories to change camp. Unless they have loads-a-money
For those buying their first DSLR it's a smuch choosing a brand to go with as choosing a camera since the differences between specs is very small these days.