My vote is definately "elliptical".

I have issues that I can not do impactive stuff exercise wise. Thus one reason why I pedal. Hard walking actually even bothers me a bit. To be honest, I don't enjoy walking either.

I do the elliptical when we are traveling in hotels (read: no pedaling options). I think it's a pretty good work out with no impact stress.

I think to be a good power walker you have to really be disciplined. In regards to your arms and pumping them for the cardio boost.

On the ellipital machine, the arms are constant