I have both Trimble GPS and Gaia GPS. Gaia GPS hands down is the winner. Just be sure to have all your maps cached before hand in either case. Though both aps have crashed on my on longer trips. Gaia seems to crash more when it is set to have direction of travel as up instead of north is up.
I use it in conjuntion with the nav system in my jeep.
I also have Waze for on the road. It has saved me lots of $$$ in tickets thanks to other people updating it real time with speed trap locations.
Google Earth is kind of fun and helps with planning longer routes. I've also used it to help plan camp sites because you can see the terrain and how isolated it might be. But it is nowhere near as useful as the desktop version.
There are a few other aps out there that are designed to work with an external GPS as well, although I've never had a problem with the GPS that my ipad has.
Right now I have an iPad Mini mounted on a Ram Mount attached to the grab bar. This is MUCH better than using a suction cup mount in the Window. Once I get around to getting a bracket for my full sized iPad I will probably switch to that.