I haven't known modern Nissans to have anything to do with Alfa (and I have a '91 Alfa 164S in storage). The head gasket problem I mentioned occured on the later generation Altima (2002 in our case, and it was a four cylinder). It is not a common occurrance, but it does apparently occur. Oil smoking seems to be a rare thing on a modern engine, and modern piston rings don't seem to wear out. Oil smoking on a modern car is more likely a result of an overheat in which the valve seals were melted.
Tone - don't get me wrong, a Honda 6 cylinder is pretty good. We just find the 4 cylinders to be a bit more reliable, less complex, a lot easier to work on, and as having enough power. We find the six cylinder Hondas/Acuras to have a bit more "Check Engine" light sensitivity as they age, and a timing belt change is more involved (when the time comes) than on a 4 cylinder. I don't think you can make a mistake with either the Altima or the Accord. As cars go, they are both a the top of the list in terms of quality, reliability, and longevity. Look at them both, see what you like, and see how either fits your budget.