So I run a Lowrance HDS 5m Gen2 with a Topo Map software package, as well as MoitionX on my iPad and iPhone.
If you pre-position maps on the iDevices, then cellular signal isn't required. I'd say the built in GPS of the full size iPad and iPhone is not as accurate as the Lowrance.
I really like the physical buttons on the Lowrance. I can manipulate them without looking where as on the iDevice I have to pinch and zoom or hunt and peck for a virtual button which can be tough while you're driving and off road.
I will say, with my topo map overlay, the route is washed out when I come to a somewhat major road. Because I can't change the width of the track I'm following, it makes it tough to see when there is a road/highway over the top. I either pop the SD card out or rely on my iPhone.
Yes the Lowrance is purpose built and expensive, but I have no regrets after pre-running and leading a group for SnoBall II.