TomTom GPS vs Garmin

[Offroad]

I'm glad to see this post.....I ran into problems using my Android when riding east and south of Knoxville, TN in the national parks there, including of course, the dragon, skyway etc. I mounted a water resistant 12 volt / 5 volt USB on my handle bars and ran the wires under my tank to my battery. Worked out great except when out of cell phone tower range.....so I was wondering if a truly GPS unit would work up in those hills and mountains?
I use a "Riggs-Nelson" tank bag I saw first on this forum that has a phone and GPS tilt up zipper pouch with a clear window that works pretty good...I could see it pretty easily when riding, but sometimes I would be happy to pull off just to see where I was at.
I was lost once going up out of Gatlinburg, TN (it was a STILL a nice 60 mile ride on extreme curvy roads) and the cell phone was not much help....a GPS unit would have been nice!