chrome will come clean (and a lot brighter) with just a quick wipe, where as polished will need a couple of hours work quite frequently.....there is always a slight risk of warping the manifold when its plated, but it is only a small risk.. the PITA with re chroming wheels is not the chroming, but the chemical stripping of the old chrome..
just convinience really, depends how much work you want to put in keeping it shiny...