But where are we talking about doing this? In the attic? And what do you have for 'existing insulation'? What do YOU think about 1 vs. the other? And at the expense of me sounding silly, why do you want to do what you're asking? What part of the county do you live? And can you say more about the house, especially the attic above (but don't forget to say a little about the rest of the house)?
RE: Wrapping the HWH - Unless it's in an unheated and uninsulated room, I don't think that adding the wrap will help that much.