To be clear, you only need an external GPS with the non-3G model. There has been extensive analysis on this forum regarding the viability of the built-in GPS in the 3G model. I've now used mine extensively in an area very unfriendly to GPS signal reception, dense tree cover in mountainous Vermont, and have enjoyed, according to my HEMA map app, 16' accuracy almost continuously. Don't know about you, but that's good enough for me.
Regarding apps, I've found HEMA to be superior to any of the other apps due mostly to additional features and data provided on the primary screen. Also, very large areas can be set to download automatically, I can't believe you'd spend more than an hour downloading topo coverage for a few states if you needed it.
CURRENT RIG: 1989 Toyota FJ62, 4" MAF lift + shackle reversal, 37" IROKs and custom trimming, Warn 12K winch, ARB front, SROR custom rear bumper + sliders, Longfields, ARB lockers F+R, 4.56 R+P. Custom rear disk brake conversion. Ghetto fabulous bumper mounted chainsaw scabbard.
SOLD: 1970 Mercedes Benz Unimog 416 DOKA, 44" XM-47s, hydraulics, tipper, PTO, 20 sp trans.