I'm not sure about the HPX being wider, but maybe that's true? I think one of the main benefits is the HPX did away with the MiniDV drive, so the camera is smaller and lighter. The HPX toggle stick for navigating the menu/clips sucks in my opinion, the button system on the HVX is a lot easier to use.
If you don't already have an MK1 or Xtreme, it might make sense to upgrade the camera before getting the lens. However, I would recommend you consider the AC160 or the HPX250, as these cameras can film 1080/60P instead of the 720P of the HVX200/HPX170.