Just got an iPad2, want to rig it as a GPS backup. Running into the same questions.
Explain this please:
Navionics HD comes with gold enc maps. iNavX come with NOAA raster images and one must pay much more extra for navionics enc maps for iNavX.
Neither website was very clear on whether I had to then turn around and pay extra for charts to be able to use the app off the shelf. Having dealt with Garmin (ugh) I know that's where they really get you, money wise.