The cost for value.
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. I think iNavX is a great app, and I may get it down the road, but I will likely get MacENC first. I currently also use GPSNavX on a cheap little laptop in my navstation, and I have been very pleased with it. So for now, the combo is great (though I hardly look at the laptop now).