Actually the amount of data a smartphone mapping app uses is relatively low and the GPS "data" is free. I wouldn't think it's a issue unless you have a REALLY low data limit plan on your phone.

IMO, the main difference is map availability. If your using a smartphone and your far from cell towers and off network you cannot download map data where the GPS has ALL the map data (from the latest update) on the device all the time. You can re-route with a dedicated GPS if you need to detour or you get lost AND your off network with your phone.